feat(notify): add percentage threshold type for balance low notification

- Add threshold_type field (fixed/percentage) to system and user settings
- Add total_recharged field to users table, auto-incremented on balance credit
- Percentage mode: effective threshold = total_recharged × percentage / 100
- User-level threshold_type inherits from system default when not set
- Update admin settings UI with radio selector (fixed amount / percentage)
- Migration: 102_add_balance_notify_threshold_type.sql

backend/internal/service/user.go

@@ -32,8 +32,10 @@ type User struct {
 
 	// 余额不足通知
 	BalanceNotifyEnabled       bool
+	BalanceNotifyThresholdType string   // "fixed" (default) | "percentage"
 	BalanceNotifyThreshold     *float64
 	BalanceNotifyExtraEmails   []string
+	TotalRecharged             float64
 
 	APIKeys       []APIKey
 	Subscriptions []UserSubscription

backend/internal/service/user_service.go

@@ -66,6 +66,7 @@ type UpdateProfileRequest struct {
 	Username                   *string  `json:"username"`
 	Concurrency                *int     `json:"concurrency"`
 	BalanceNotifyEnabled       *bool    `json:"balance_notify_enabled"`
+	BalanceNotifyThresholdType *string  `json:"balance_notify_threshold_type"`
 	BalanceNotifyThreshold     *float64 `json:"balance_notify_threshold"`
 }
 
@@ -143,6 +144,9 @@ func (s *UserService) UpdateProfile(ctx context.Context, userID int64, req Updat
 	if req.BalanceNotifyEnabled != nil {
 		user.BalanceNotifyEnabled = *req.BalanceNotifyEnabled
 	}
+	if req.BalanceNotifyThresholdType != nil {
+		user.BalanceNotifyThresholdType = *req.BalanceNotifyThresholdType
+	}
 	if req.BalanceNotifyThreshold != nil {
 		if *req.BalanceNotifyThreshold <= 0 {
 			user.BalanceNotifyThreshold = nil // clear to system default

backend/migrations/102_add_balance_notify_threshold_type.sql

+-- Add threshold type support (fixed / percentage) to balance notification
+ALTER TABLE users ADD COLUMN IF NOT EXISTS balance_notify_threshold_type VARCHAR(10) NOT NULL DEFAULT 'fixed';
+-- Track cumulative recharge amount for percentage threshold calculation
+ALTER TABLE users ADD COLUMN IF NOT EXISTS total_recharged DECIMAL(20,8) NOT NULL DEFAULT 0;

frontend/src/api/admin/settings.ts

@@ -137,6 +137,7 @@ export interface SystemSettings {
 
   // Balance & quota notification
   balance_low_notify_enabled: boolean
+  balance_low_notify_threshold_type: 'fixed' | 'percentage'
   balance_low_notify_threshold: number
   account_quota_notify_emails: string[]
 }
@@ -240,6 +241,7 @@ export interface UpdateSettingsRequest {
   payment_cancel_rate_limit_window_mode?: string
   // Balance & quota notification
   balance_low_notify_enabled?: boolean
+  balance_low_notify_threshold_type?: 'fixed' | 'percentage'
   balance_low_notify_threshold?: number
   account_quota_notify_emails?: string[]
 }

frontend/src/i18n/locales/en.ts

@@ -4633,8 +4633,12 @@ export default {
         title: 'Balance Low Notification',
         description: 'Send email notification when user balance falls below threshold',
         enabled: 'Enable Balance Low Notification',
+        thresholdType: 'Threshold Type',
+        typeFixed: 'Fixed Amount',
+        typePercentage: 'Percentage of Recharged',
         threshold: 'Default Threshold',
         thresholdHint: 'Used when user has not set a custom value',
+        percentageHint: 'Notify when balance falls below this percentage of total recharged amount',
         thresholdPlaceholder: 'Enter amount',
       },
       quotaNotify: {

frontend/src/i18n/locales/zh.ts

@@ -4797,8 +4797,12 @@ export default {
         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',
+        thresholdType: '阈值类型',
+        typeFixed: '固定金额',
+        typePercentage: '充值百分比',
+        threshold: '提醒阈值',
         thresholdHint: '用户未自定义时使用此值',
+        percentageHint: '当余额低于累计充值额的此百分比时提醒',
         thresholdPlaceholder: '输入金额',
       },
       quotaNotify: {

frontend/src/views/admin/SettingsView.vue

@@ -2660,6 +2660,90 @@
             </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" class="space-y-3">
+              <!-- Threshold type selector -->
+              <div>
+                <label class="mb-2 block text-sm font-medium text-gray-700 dark:text-gray-300">{{ t('admin.settings.balanceNotify.thresholdType') }}</label>
+                <div class="flex gap-4">
+                  <label class="flex items-center gap-1.5 text-sm cursor-pointer">
+                    <input type="radio" v-model="form.balance_low_notify_threshold_type" value="fixed" class="accent-primary-500" />
+                    {{ t('admin.settings.balanceNotify.typeFixed') }}
+                  </label>
+                  <label class="flex items-center gap-1.5 text-sm cursor-pointer">
+                    <input type="radio" v-model="form.balance_low_notify_threshold_type" value="percentage" class="accent-primary-500" />
+                    {{ t('admin.settings.balanceNotify.typePercentage') }}
+                  </label>
+                </div>
+              </div>
+              <!-- Threshold value -->
+              <div>
+                <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">
+                    {{ form.balance_low_notify_threshold_type === 'percentage' ? '%' : '$' }}
+                  </span>
+                  <input
+                    v-model.number="form.balance_low_notify_threshold"
+                    type="number"
+                    :min="0"
+                    :max="form.balance_low_notify_threshold_type === 'percentage' ? 100 : undefined"
+                    :step="form.balance_low_notify_threshold_type === 'percentage' ? 1 : 0.01"
+                    class="input pl-7"
+                  />
+                </div>
+                <p class="mt-1 text-xs text-gray-500 dark:text-gray-400">
+                  {{ form.balance_low_notify_threshold_type === 'percentage'
+                    ? t('admin.settings.balanceNotify.percentageHint')
+                    : t('admin.settings.balanceNotify.thresholdHint') }}
+                </p>
+              </div>
+            </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 -->
@@ -2939,7 +3023,12 @@ 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_type: 'fixed' as 'fixed' | 'percentage',
+  balance_low_notify_threshold: 0,
+  account_quota_notify_emails: [] as string[]
 })
 
 // Proxies for web search emulation ProxySelector
@@ -3149,6 +3238,14 @@ 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 ''
@@ -3488,6 +3585,11 @@ 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_type: form.balance_low_notify_threshold_type,
+      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)