feat(notify): add balance low & account quota notification system

- User balance low notification: email alert when balance drops below
  configurable threshold (user email + verified extra emails)
- Account quota notification: broadcast email to admin-configured
  recipients when daily/weekly/total quota usage exceeds alert threshold
- Admin settings: global enable/disable, default threshold, quota
  notification email list (Email Settings tab)
- User profile: enable/disable, custom threshold, add/remove extra
  notification emails with verification code flow
- Account quota: per-dimension alert toggle and threshold in quota
  control card
- Trigger logic: first-crossing only (old >= threshold && new < threshold
  for balance; old < threshold && new >= threshold for quota), naturally
  prevents duplicate notifications without Redis dedup

frontend/src/components/account/QuotaLimitCard.vue

@@ -4,7 +4,7 @@ import { useI18n } from 'vue-i18n'
 
 const { t } = useI18n()
 
-const props = defineProps<{
+const props = withDefaults(defineProps<{
   totalLimit: number | null
   dailyLimit: number | null
   weeklyLimit: number | null
@@ -14,7 +14,20 @@ const props = defineProps<{
   weeklyResetDay: number | null
   weeklyResetHour: number | null
   resetTimezone: string | null
-}>()
+  quotaNotifyDailyEnabled?: boolean | null
+  quotaNotifyDailyThreshold?: number | null
+  quotaNotifyWeeklyEnabled?: boolean | null
+  quotaNotifyWeeklyThreshold?: number | null
+  quotaNotifyTotalEnabled?: boolean | null
+  quotaNotifyTotalThreshold?: number | null
+}>(), {
+  quotaNotifyDailyEnabled: null,
+  quotaNotifyDailyThreshold: null,
+  quotaNotifyWeeklyEnabled: null,
+  quotaNotifyWeeklyThreshold: null,
+  quotaNotifyTotalEnabled: null,
+  quotaNotifyTotalThreshold: null,
+})
 
 const emit = defineEmits<{
   'update:totalLimit': [value: number | null]
@@ -26,6 +39,12 @@ const emit = defineEmits<{
   'update:weeklyResetDay': [value: number | null]
   'update:weeklyResetHour': [value: number | null]
   'update:resetTimezone': [value: string | null]
+  'update:quotaNotifyDailyEnabled': [value: boolean | null]
+  'update:quotaNotifyDailyThreshold': [value: number | null]
+  'update:quotaNotifyWeeklyEnabled': [value: boolean | null]
+  'update:quotaNotifyWeeklyThreshold': [value: number | null]
+  'update:quotaNotifyTotalEnabled': [value: boolean | null]
+  'update:quotaNotifyTotalThreshold': [value: number | null]
 }>()
 
 const enabled = computed(() =>
@@ -203,6 +222,36 @@ const onWeeklyModeChange = (e: Event) => {
               {{ t('admin.accounts.quotaDailyLimitHint') }}
             </template>
           </p>
+          <!-- 日配额告警 -->
+          <div v-if="dailyLimit && dailyLimit > 0" class="ml-4 mt-2 flex items-center gap-3">
+            <label class="text-sm text-gray-500 whitespace-nowrap">{{ t('admin.accounts.quotaNotify.alert') }}</label>
+            <button
+              type="button"
+              @click="emit('update:quotaNotifyDailyEnabled', !(props.quotaNotifyDailyEnabled))"
+              :class="[
+                'relative inline-flex h-5 w-9 flex-shrink-0 cursor-pointer rounded-full border-2 border-transparent transition-colors duration-200 ease-in-out focus:outline-none',
+                props.quotaNotifyDailyEnabled ? 'bg-primary-600' : 'bg-gray-200 dark:bg-dark-600'
+              ]"
+            >
+              <span
+                :class="[
+                  'pointer-events-none inline-block h-4 w-4 transform rounded-full bg-white shadow ring-0 transition duration-200 ease-in-out',
+                  props.quotaNotifyDailyEnabled ? 'translate-x-4' : 'translate-x-0'
+                ]"
+              />
+            </button>
+            <div v-if="props.quotaNotifyDailyEnabled" class="relative flex-1">
+              <span class="absolute left-2 top-1/2 -translate-y-1/2 text-gray-400 text-sm">$</span>
+              <input
+                :value="props.quotaNotifyDailyThreshold"
+                @input="emit('update:quotaNotifyDailyThreshold', parseFloat(($event.target as HTMLInputElement).value) || null)"
+                type="number"
+                min="0"
+                step="0.01"
+                class="input pl-6 py-1 text-sm"
+              />
+            </div>
+          </div>
         </div>
 
         <!-- 周配额 -->
@@ -259,6 +308,36 @@ const onWeeklyModeChange = (e: Event) => {
               {{ t('admin.accounts.quotaWeeklyLimitHint') }}
             </template>
           </p>
+          <!-- 周配额告警 -->
+          <div v-if="weeklyLimit && weeklyLimit > 0" class="ml-4 mt-2 flex items-center gap-3">
+            <label class="text-sm text-gray-500 whitespace-nowrap">{{ t('admin.accounts.quotaNotify.alert') }}</label>
+            <button
+              type="button"
+              @click="emit('update:quotaNotifyWeeklyEnabled', !(props.quotaNotifyWeeklyEnabled))"
+              :class="[
+                'relative inline-flex h-5 w-9 flex-shrink-0 cursor-pointer rounded-full border-2 border-transparent transition-colors duration-200 ease-in-out focus:outline-none',
+                props.quotaNotifyWeeklyEnabled ? 'bg-primary-600' : 'bg-gray-200 dark:bg-dark-600'
+              ]"
+            >
+              <span
+                :class="[
+                  'pointer-events-none inline-block h-4 w-4 transform rounded-full bg-white shadow ring-0 transition duration-200 ease-in-out',
+                  props.quotaNotifyWeeklyEnabled ? 'translate-x-4' : 'translate-x-0'
+                ]"
+              />
+            </button>
+            <div v-if="props.quotaNotifyWeeklyEnabled" class="relative flex-1">
+              <span class="absolute left-2 top-1/2 -translate-y-1/2 text-gray-400 text-sm">$</span>
+              <input
+                :value="props.quotaNotifyWeeklyThreshold"
+                @input="emit('update:quotaNotifyWeeklyThreshold', parseFloat(($event.target as HTMLInputElement).value) || null)"
+                type="number"
+                min="0"
+                step="0.01"
+                class="input pl-6 py-1 text-sm"
+              />
+            </div>
+          </div>
         </div>
 
         <!-- 时区选择（当任一维度使用固定模式时显示） -->
@@ -289,6 +368,36 @@ const onWeeklyModeChange = (e: Event) => {
             />
           </div>
           <p class="input-hint">{{ t('admin.accounts.quotaTotalLimitHint') }}</p>
+          <!-- 总配额告警 -->
+          <div v-if="totalLimit && totalLimit > 0" class="ml-4 mt-2 flex items-center gap-3">
+            <label class="text-sm text-gray-500 whitespace-nowrap">{{ t('admin.accounts.quotaNotify.alert') }}</label>
+            <button
+              type="button"
+              @click="emit('update:quotaNotifyTotalEnabled', !(props.quotaNotifyTotalEnabled))"
+              :class="[
+                'relative inline-flex h-5 w-9 flex-shrink-0 cursor-pointer rounded-full border-2 border-transparent transition-colors duration-200 ease-in-out focus:outline-none',
+                props.quotaNotifyTotalEnabled ? 'bg-primary-600' : 'bg-gray-200 dark:bg-dark-600'
+              ]"
+            >
+              <span
+                :class="[
+                  'pointer-events-none inline-block h-4 w-4 transform rounded-full bg-white shadow ring-0 transition duration-200 ease-in-out',
+                  props.quotaNotifyTotalEnabled ? 'translate-x-4' : 'translate-x-0'
+                ]"
+              />
+            </button>
+            <div v-if="props.quotaNotifyTotalEnabled" class="relative flex-1">
+              <span class="absolute left-2 top-1/2 -translate-y-1/2 text-gray-400 text-sm">$</span>
+              <input
+                :value="props.quotaNotifyTotalThreshold"
+                @input="emit('update:quotaNotifyTotalThreshold', parseFloat(($event.target as HTMLInputElement).value) || null)"
+                type="number"
+                min="0"
+                step="0.01"
+                class="input pl-6 py-1 text-sm"
+              />
+            </div>
+          </div>
         </div>
       </div>
   </div>

frontend/src/components/user/profile/ProfileBalanceNotifyCard.vue

+<template>
+  <div class="card">
+    <div class="border-b border-gray-100 px-6 py-4 dark:border-dark-700">
+      <h2 class="text-lg font-medium text-gray-900 dark:text-white">
+        {{ t('profile.balanceNotify.title') }}
+      </h2>
+    </div>
+    <div class="px-6 py-6 space-y-6">
+      <!-- Enable toggle -->
+      <div class="flex items-center justify-between">
+        <div>
+          <label class="input-label">{{ t('profile.balanceNotify.enabled') }}</label>
+        </div>
+        <label class="relative inline-flex items-center cursor-pointer">
+          <input type="checkbox" v-model="notifyEnabled" @change="handleToggle" class="sr-only peer" />
+          <div class="w-11 h-6 bg-gray-200 peer-focus:outline-none peer-focus:ring-4 peer-focus:ring-primary-300 dark:peer-focus:ring-primary-800 rounded-full peer dark:bg-gray-700 peer-checked:after:translate-x-full peer-checked:after:border-white after:content-[''] after:absolute after:top-[2px] after:left-[2px] after:bg-white after:border-gray-300 after:border after:rounded-full after:h-5 after:w-5 after:transition-all dark:after:border-gray-600 peer-checked:bg-primary-600"></div>
+        </label>
+      </div>
+
+      <!-- Custom threshold -->
+      <div v-if="notifyEnabled">
+        <label class="input-label">
+          {{ t('profile.balanceNotify.threshold') }}
+          <span class="text-xs text-gray-400 ml-2">{{ t('profile.balanceNotify.thresholdHint') }}</span>
+        </label>
+        <div class="flex items-center gap-2">
+          <span class="text-gray-500">$</span>
+          <input
+            v-model.number="customThreshold"
+            type="number"
+            min="0"
+            step="0.01"
+            class="input flex-1"
+            :placeholder="t('profile.balanceNotify.thresholdPlaceholder')"
+            @blur="handleThresholdUpdate"
+          />
+        </div>
+      </div>
+
+      <!-- Extra emails -->
+      <div v-if="notifyEnabled">
+        <label class="input-label">{{ t('profile.balanceNotify.extraEmails') }}</label>
+
+        <!-- Existing emails list -->
+        <div v-if="extraEmails.length > 0" class="space-y-2 mb-4">
+          <div v-for="email in extraEmails" :key="email"
+            class="flex items-center justify-between px-3 py-2 bg-gray-50 dark:bg-dark-700 rounded-lg">
+            <span class="text-sm text-gray-700 dark:text-gray-300">{{ email }}</span>
+            <button @click="handleRemoveEmail(email)" class="text-red-500 hover:text-red-700 text-sm">
+              {{ t('profile.balanceNotify.removeEmail') }}
+            </button>
+          </div>
+        </div>
+
+        <!-- Add new email -->
+        <div class="space-y-2">
+          <div class="flex gap-2">
+            <input
+              v-model="newEmail"
+              type="email"
+              class="input flex-1"
+              :placeholder="t('profile.balanceNotify.emailPlaceholder')"
+              :disabled="codeSent"
+            />
+            <button
+              @click="handleSendCode"
+              :disabled="!newEmail || sendingCode || codeCountdown > 0"
+              class="btn btn-outline whitespace-nowrap"
+            >
+              {{ codeCountdown > 0 ? `${codeCountdown}s` : (codeSent ? t('profile.balanceNotify.codeSent') : t('profile.balanceNotify.sendCode')) }}
+            </button>
+          </div>
+          <div v-if="codeSent" class="flex gap-2">
+            <input
+              v-model="verifyCode"
+              type="text"
+              maxlength="6"
+              class="input flex-1"
+              :placeholder="t('profile.balanceNotify.codePlaceholder')"
+            />
+            <button
+              @click="handleVerify"
+              :disabled="!verifyCode || verifyCode.length !== 6 || verifying"
+              class="btn btn-primary whitespace-nowrap"
+            >
+              {{ t('profile.balanceNotify.verify') }}
+            </button>
+          </div>
+        </div>
+      </div>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { ref, watch } from 'vue'
+import { useI18n } from 'vue-i18n'
+import { useAuthStore } from '@/stores/auth'
+import { useAppStore } from '@/stores/app'
+import { userAPI } from '@/api'
+import { extractApiErrorMessage } from '@/utils/apiError'
+
+const props = defineProps<{
+  enabled: boolean
+  threshold: number | null
+  extraEmails: string[]
+}>()
+
+const { t } = useI18n()
+const authStore = useAuthStore()
+const appStore = useAppStore()
+
+const notifyEnabled = ref(props.enabled)
+const customThreshold = ref<number | null>(props.threshold)
+const extraEmails = ref<string[]>([...props.extraEmails])
+const newEmail = ref('')
+const verifyCode = ref('')
+const codeSent = ref(false)
+const sendingCode = ref(false)
+const verifying = ref(false)
+const codeCountdown = ref(0)
+
+let countdownTimer: ReturnType<typeof setInterval> | null = null
+
+watch(() => props.enabled, (val) => { notifyEnabled.value = val })
+watch(() => props.threshold, (val) => { customThreshold.value = val })
+watch(() => props.extraEmails, (val) => { extraEmails.value = [...val] })
+
+const handleToggle = async () => {
+  try {
+    const updated = await userAPI.updateProfile({ balance_notify_enabled: notifyEnabled.value })
+    authStore.user = updated
+  } catch (err: unknown) {
+    appStore.showError(extractApiErrorMessage(err, t('common.error')))
+    notifyEnabled.value = !notifyEnabled.value
+  }
+}
+
+const handleThresholdUpdate = async () => {
+  try {
+    const threshold = customThreshold.value && customThreshold.value > 0 ? customThreshold.value : 0
+    const updated = await userAPI.updateProfile({ balance_notify_threshold: threshold })
+    authStore.user = updated
+  } catch (err: unknown) {
+    appStore.showError(extractApiErrorMessage(err, t('common.error')))
+  }
+}
+
+const handleSendCode = async () => {
+  if (!newEmail.value) return
+  sendingCode.value = true
+  try {
+    await userAPI.sendNotifyEmailCode(newEmail.value)
+    codeSent.value = true
+    codeCountdown.value = 60
+    countdownTimer = setInterval(() => {
+      codeCountdown.value--
+      if (codeCountdown.value <= 0) {
+        if (countdownTimer) clearInterval(countdownTimer)
+        countdownTimer = null
+      }
+    }, 1000)
+    appStore.showSuccess(t('profile.balanceNotify.codeSent'))
+  } catch (err: unknown) {
+    appStore.showError(extractApiErrorMessage(err, t('common.error')))
+  } finally {
+    sendingCode.value = false
+  }
+}
+
+const handleVerify = async () => {
+  if (!verifyCode.value || verifyCode.value.length !== 6) return
+  verifying.value = true
+  try {
+    await userAPI.verifyNotifyEmail(newEmail.value, verifyCode.value)
+    extraEmails.value.push(newEmail.value)
+    newEmail.value = ''
+    verifyCode.value = ''
+    codeSent.value = false
+    if (countdownTimer) clearInterval(countdownTimer)
+    codeCountdown.value = 0
+    appStore.showSuccess(t('profile.balanceNotify.verifySuccess'))
+    // Refresh user data
+    const updated = await userAPI.getProfile()
+    authStore.user = updated
+  } catch (err: unknown) {
+    appStore.showError(extractApiErrorMessage(err, t('common.error')))
+  } finally {
+    verifying.value = false
+  }
+}
+
+const handleRemoveEmail = async (email: string) => {
+  try {
+    await userAPI.removeNotifyEmail(email)
+    extraEmails.value = extraEmails.value.filter(e => e !== email)
+    appStore.showSuccess(t('profile.balanceNotify.removeSuccess'))
+    const updated = await userAPI.getProfile()
+    authStore.user = updated
+  } catch (err: unknown) {
+    appStore.showError(extractApiErrorMessage(err, t('common.error')))
+  }
+}
+</script>

frontend/src/i18n/locales/en.ts

@@ -902,6 +902,31 @@ export default {
       sendCode: 'Send Code',
       codeSent: 'Verification code sent to your email',
       sendCodeFailed: 'Failed to send verification code'
+    },
+    balanceNotify: {
+      title: 'Balance Low Notification',
+      description: 'Send email alert when account balance falls below threshold',
+      enabled: 'Enable Balance Low Notification',
+      threshold: 'Custom Threshold',
+      thresholdHint: 'Leave empty to use system default',
+      thresholdPlaceholder: 'Enter amount',
+      systemDefault: 'System Default',
+      extraEmails: 'Extra Notification Emails',
+      noExtraEmails: 'No extra notification emails',
+      enterEmail: 'Enter email address',
+      addEmail: 'Add Email',
+      emailPlaceholder: 'Enter email address',
+      sendCode: 'Send Code',
+      codeSent: 'Verification code sent',
+      codeSentTo: 'Code sent to {email}',
+      enterCode: 'Enter verification code',
+      codePlaceholder: '6-digit code',
+      verify: 'Verify & Add',
+      emailAdded: 'Email added',
+      emailRemoved: 'Email removed',
+      verifySuccess: 'Email added successfully',
+      removeEmail: 'Remove',
+      removeSuccess: 'Email removed',
     }
   },
 
@@ -2228,6 +2253,12 @@ export default {
       },
       quotaLimitAmount: 'Total Limit',
       quotaLimitAmountHint: 'Cumulative spending limit. Does not auto-reset.',
+      quotaNotify: {
+        alert: 'Alert Threshold',
+        enabled: 'Enable Alert',
+        threshold: 'Alert Amount',
+        thresholdPlaceholder: 'Enter alert amount',
+      },
       testConnection: 'Test Connection',
       reAuthorize: 'Re-Authorize',
       refreshToken: 'Refresh Token',
@@ -4593,6 +4624,22 @@ export default {
         supportedTypesHint: 'Comma-separated, e.g. alipay,wxpay',
         refundEnabled: 'Allow Refund',
       },
+      balanceNotify: {
+        title: 'Balance Low Notification',
+        description: 'Send email notification when user balance falls below threshold',
+        enabled: 'Enable Balance Low Notification',
+        threshold: 'Default Threshold',
+        thresholdHint: 'Used when user has not set a custom value',
+        thresholdPlaceholder: 'Enter amount',
+      },
+      quotaNotify: {
+        title: 'Account Quota Notification',
+        description: 'Notify admins when account quota usage reaches alert threshold',
+        emails: 'Notification Emails',
+        emailsHint: 'Leave empty to disable notifications',
+        addEmail: 'Add Email',
+        emailPlaceholder: 'Enter email address',
+      },
       smtp: {
         title: 'SMTP Settings',
         description: 'Configure email sending for verification codes',

frontend/src/i18n/locales/zh.ts

@@ -906,6 +906,31 @@ export default {
       sendCode: '发送验证码',
       codeSent: '验证码已发送到您的邮箱',
       sendCodeFailed: '发送验证码失败'
+    },
+    balanceNotify: {
+      title: '余额不足提醒',
+      description: '当账户余额低于阈值时发送邮件提醒',
+      enabled: '启用余额不足提醒',
+      threshold: '自定义提醒阈值',
+      thresholdHint: '留空使用系统默认值',
+      thresholdPlaceholder: '输入金额',
+      systemDefault: '系统默认值',
+      extraEmails: '额外通知邮箱',
+      noExtraEmails: '暂无额外通知邮箱',
+      enterEmail: '输入邮箱地址',
+      addEmail: '添加邮箱',
+      emailPlaceholder: '输入邮箱地址',
+      sendCode: '发送验证码',
+      codeSent: '验证码已发送',
+      codeSentTo: '验证码已发送到 {email}',
+      enterCode: '输入验证码',
+      codePlaceholder: '6位验证码',
+      verify: '确认添加',
+      emailAdded: '邮箱已添加',
+      emailRemoved: '邮箱已移除',
+      verifySuccess: '邮箱添加成功',
+      removeEmail: '移除',
+      removeSuccess: '邮箱已移除',
     }
   },
 
@@ -2226,6 +2251,12 @@ export default {
       },
       quotaLimitAmount: '总限额',
       quotaLimitAmountHint: '累计消费上限，不会自动重置。',
+      quotaNotify: {
+        alert: '告警阈值',
+        enabled: '启用告警',
+        threshold: '告警金额',
+        thresholdPlaceholder: '输入告警金额',
+      },
       testConnection: '测试连接',
       reAuthorize: '重新授权',
       refreshToken: '刷新令牌',
@@ -4757,6 +4788,22 @@ export default {
         supportedTypesHint: '逗号分隔，如 alipay,wxpay',
         refundEnabled: '允许退款',
       },
+      balanceNotify: {
+        title: '余额不足提醒',
+        description: '当用户余额低于阈值时发送邮件提醒',
+        enabled: '启用余额不足提醒',
+        threshold: '默认提醒阈值',
+        thresholdHint: '用户未自定义时使用此值',
+        thresholdPlaceholder: '输入金额',
+      },
+      quotaNotify: {
+        title: '账号限额通知',
+        description: '当账号配额用量达到告警阈值时通知管理员',
+        emails: '通知邮箱',
+        emailsHint: '留空则不发送通知',
+        addEmail: '添加邮箱',
+        emailPlaceholder: '输入邮箱地址',
+      },
       smtp: {
         title: 'SMTP 设置',
         description: '配置用于发送验证码的邮件服务',

frontend/src/types/index.ts

@@ -33,6 +33,9 @@ export interface User {
   concurrency: number // Allowed concurrent requests
   status: 'active' | 'disabled' // Account status
   allowed_groups: number[] | null // Allowed group IDs (null = all non-exclusive groups)
+  balance_notify_enabled: boolean
+  balance_notify_threshold: number | null
+  balance_notify_extra_emails: string[]
   subscriptions?: UserSubscription[] // User's active subscriptions
   created_at: string
   updated_at: string

frontend/src/views/admin/SettingsView.vue

@@ -2562,6 +2562,60 @@
             </div>
           </div>
         </div>
+        <!-- Balance Low Notification -->
+        <div class="card">
+          <div class="border-b border-gray-100 px-6 py-4 dark:border-dark-700">
+            <h3 class="text-base font-medium text-gray-900 dark:text-white">
+              {{ t('admin.settings.balanceNotify.title') }}
+            </h3>
+            <p class="mt-1 text-sm text-gray-500 dark:text-gray-400">
+              {{ t('admin.settings.balanceNotify.description') }}
+            </p>
+          </div>
+          <div class="px-6 py-6 space-y-4">
+            <div class="flex items-center justify-between">
+              <label class="mb-0 block text-sm font-medium text-gray-700 dark:text-gray-300">{{ t('admin.settings.balanceNotify.enabled') }}</label>
+              <Toggle v-model="form.balance_low_notify_enabled" />
+            </div>
+            <div v-if="form.balance_low_notify_enabled">
+              <label class="mb-2 block text-sm font-medium text-gray-700 dark:text-gray-300">{{ t('admin.settings.balanceNotify.threshold') }}</label>
+              <div class="relative">
+                <span class="absolute left-3 top-1/2 -translate-y-1/2 text-gray-400">$</span>
+                <input v-model.number="form.balance_low_notify_threshold" type="number" min="0" step="0.01" class="input pl-7" />
+              </div>
+              <p class="mt-1 text-xs text-gray-500 dark:text-gray-400">{{ t('admin.settings.balanceNotify.thresholdHint') }}</p>
+            </div>
+          </div>
+        </div>
+
+        <!-- Account Quota Notification -->
+        <div class="card">
+          <div class="border-b border-gray-100 px-6 py-4 dark:border-dark-700">
+            <h3 class="text-base font-medium text-gray-900 dark:text-white">
+              {{ t('admin.settings.quotaNotify.title') }}
+            </h3>
+            <p class="mt-1 text-sm text-gray-500 dark:text-gray-400">
+              {{ t('admin.settings.quotaNotify.description') }}
+            </p>
+          </div>
+          <div class="px-6 py-6 space-y-4">
+            <div>
+              <label class="mb-2 block text-sm font-medium text-gray-700 dark:text-gray-300">{{ t('admin.settings.quotaNotify.emails') }}</label>
+              <div class="space-y-2">
+                <div v-for="(_, index) in (form.account_quota_notify_emails || [])" :key="index" class="flex items-center gap-2">
+                  <input v-model="form.account_quota_notify_emails[index]" type="email" class="input flex-1" />
+                  <button @click="form.account_quota_notify_emails.splice(index, 1)" class="btn btn-secondary px-2" type="button">
+                    <Icon name="x" size="xs" class="h-4 w-4" />
+                  </button>
+                </div>
+                <button @click="addQuotaNotifyEmail" class="btn btn-secondary btn-sm" type="button">
+                  + {{ t('admin.settings.quotaNotify.addEmail') }}
+                </button>
+              </div>
+              <p class="mt-1 text-xs text-gray-500 dark:text-gray-400">{{ t('admin.settings.quotaNotify.emailsHint') }}</p>
+            </div>
+          </div>
+        </div>
         </div><!-- /Tab: Email -->
 
         <!-- Tab: Backup -->
@@ -2840,7 +2894,11 @@ const form = reactive<SettingsForm>({
   // Gateway forwarding behavior
   enable_fingerprint_unification: true,
   enable_metadata_passthrough: false,
-  enable_cch_signing: false
+  enable_cch_signing: false,
+  // Balance & quota notification
+  balance_low_notify_enabled: false,
+  balance_low_notify_threshold: 0,
+  account_quota_notify_emails: [] as string[]
 })
 
 // Web Search Emulation config (loaded/saved separately)
@@ -2972,6 +3030,14 @@ function handleRegistrationEmailSuffixWhitelistPaste(event: ClipboardEvent) {
   }
 }
 
+// Quota notify email helpers
+const addQuotaNotifyEmail = () => {
+  if (!form.account_quota_notify_emails) {
+    form.account_quota_notify_emails = []
+  }
+  form.account_quota_notify_emails.push('')
+}
+
 // LinuxDo OAuth redirect URL suggestion
 const linuxdoRedirectUrlSuggestion = computed(() => {
   if (typeof window === 'undefined') return ''
@@ -3311,6 +3377,10 @@ async function saveSettings() {
       payment_cancel_rate_limit_window: Number(form.payment_cancel_rate_limit_window) || 1,
       payment_cancel_rate_limit_unit: form.payment_cancel_rate_limit_unit,
       payment_cancel_rate_limit_window_mode: form.payment_cancel_rate_limit_window_mode,
+      // Balance & quota notification
+      balance_low_notify_enabled: form.balance_low_notify_enabled,
+      balance_low_notify_threshold: Number(form.balance_low_notify_threshold) || 0,
+      account_quota_notify_emails: (form.account_quota_notify_emails || []).filter((e: string) => e.trim() !== ''),
     }
 
     const updated = await adminAPI.settings.updateSettings(payload)

frontend/src/views/user/ProfileView.vue

@@ -14,6 +14,12 @@
         </div>
       </div>
       <ProfileEditForm :initial-username="user?.username || ''" />
+      <ProfileBalanceNotifyCard
+        v-if="user"
+        :enabled="user.balance_notify_enabled ?? true"
+        :threshold="user.balance_notify_threshold"
+        :extra-emails="user.balance_notify_extra_emails ?? []"
+      />
       <ProfilePasswordForm />
       <ProfileTotpCard />
     </div>
@@ -27,6 +33,7 @@ import { authAPI } from '@/api'; import AppLayout from '@/components/layout/AppL
 import StatCard from '@/components/common/StatCard.vue'
 import ProfileInfoCard from '@/components/user/profile/ProfileInfoCard.vue'
 import ProfileEditForm from '@/components/user/profile/ProfileEditForm.vue'
+import ProfileBalanceNotifyCard from '@/components/user/profile/ProfileBalanceNotifyCard.vue'
 import ProfilePasswordForm from '@/components/user/profile/ProfilePasswordForm.vue'
 import ProfileTotpCard from '@/components/user/profile/ProfileTotpCard.vue'
 import { Icon } from '@/components/icons'