chore: remove all sora dead code and fork-specific sora_client_enabled

Upstream removed sora feature (090_drop_sora.sql) but left i18n keys
and wire.go references. Clean up:
- Remove entire sora i18n block from en.ts and zh.ts (~190 lines)
- Remove sora nav key and unused 'data' settings tab key
- Remove sora_client_enabled from settings (fork-specific)
- Remove SoraMediaCleanupService from wire.go

backend/cmd/server/wire.go

@@ -84,7 +84,6 @@ func provideCleanup(
 	opsCleanup *service.OpsCleanupService,
 	opsScheduledReport *service.OpsScheduledReportService,
 	opsSystemLogSink *service.OpsSystemLogSink,
-	soraMediaCleanup *service.SoraMediaCleanupService,
 	schedulerSnapshot *service.SchedulerSnapshotService,
 	tokenRefresh *service.TokenRefreshService,
 	accountExpiry *service.AccountExpiryService,
@@ -134,12 +133,6 @@ func provideCleanup(
 				}
 				return nil
 			}},
-			{"SoraMediaCleanupService", func() error {
-				if soraMediaCleanup != nil {
-					soraMediaCleanup.Stop()
-				}
-				return nil
-			}},
 			{"OpsAlertEvaluatorService", func() error {
 				if opsAlertEvaluator != nil {
 					opsAlertEvaluator.Stop()

frontend/src/i18n/locales/en.ts

@@ -355,7 +355,6 @@ export default {
     mySubscriptions: 'My Subscriptions',
     buySubscription: 'Recharge / Subscription',
     docs: 'Docs',
-    sora: 'Sora Studio',
     myOrders: 'My Orders',
     orderManagement: 'Orders',
     paymentDashboard: 'Payment Dashboard',
@@ -4188,7 +4187,6 @@ export default {
         gateway: 'Gateway',
         email: 'Email',
         backup: 'Backup',
-        data: 'Sora Storage',
         payment: 'Payment',
       },
       emailTabDisabledTitle: 'Email Verification Not Enabled',
@@ -5176,6 +5174,7 @@ export default {
     }
   },
 
+  // Payment System
   payment: {
     title: 'Recharge / Subscription',
     amountLabel: 'Amount',

frontend/src/i18n/locales/zh.ts

@@ -355,7 +355,6 @@ export default {
     mySubscriptions: '我的订阅',
     buySubscription: '充值/订阅',
     docs: '文档',
-    sora: 'Sora 创作',
     myOrders: '我的订单',
     orderManagement: '订单管理',
     paymentDashboard: '支付概览',
@@ -4353,7 +4352,6 @@ export default {
         gateway: '网关服务',
         email: '邮件设置',
         backup: '数据备份',
-        data: 'Sora 存储',
         payment: '支付设置',
       },
       emailTabDisabledTitle: '邮箱验证未启用',
@@ -5364,6 +5362,7 @@ export default {
     }
   },
 
+  // Payment System
   payment: {
     title: '充值/订阅',
     amountLabel: '充值金额',