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

backend/internal/service/openai_gateway_service.go

@@ -327,6 +327,7 @@ type OpenAIGatewayService struct {
 	openaiWSResolver      OpenAIWSProtocolResolver
 	resolver              *ModelPricingResolver
 	channelService        *ChannelService
+	balanceNotifyService  *BalanceNotifyService
 
 	openaiWSPoolOnce              sync.Once
 	openaiWSStateStoreOnce        sync.Once
@@ -364,6 +365,7 @@ func NewOpenAIGatewayService(
 	openAITokenProvider *OpenAITokenProvider,
 	resolver *ModelPricingResolver,
 	channelService *ChannelService,
+	balanceNotifyService *BalanceNotifyService,
 ) *OpenAIGatewayService {
 	svc := &OpenAIGatewayService{
 		accountRepo:         accountRepo,
@@ -393,6 +395,7 @@ func NewOpenAIGatewayService(
 		openaiWSResolver:      NewOpenAIWSProtocolResolver(cfg),
 		resolver:              resolver,
 		channelService:        channelService,
+		balanceNotifyService:  balanceNotifyService,
 		responseHeaderFilter:  compileResponseHeaderFilter(cfg),
 		codexSnapshotThrottle: newAccountWriteThrottle(openAICodexSnapshotPersistMinInterval),
 	}
@@ -477,11 +480,12 @@ func (s *OpenAIGatewayService) getCodexSnapshotThrottle() *accountWriteThrottle
 
 func (s *OpenAIGatewayService) billingDeps() *billingDeps {
 	return &billingDeps{
-		accountRepo:         s.accountRepo,
-		userRepo:            s.userRepo,
-		userSubRepo:         s.userSubRepo,
-		billingCacheService: s.billingCacheService,
-		deferredService:     s.deferredService,
+		accountRepo:          s.accountRepo,
+		userRepo:             s.userRepo,
+		userSubRepo:          s.userSubRepo,
+		billingCacheService:  s.billingCacheService,
+		deferredService:      s.deferredService,
+		balanceNotifyService: s.balanceNotifyService,
 	}
 }