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/wire.go

@@ -465,6 +465,7 @@ var ProviderSet = wire.NewSet(
 	ProvidePaymentConfigService,
 	NewPaymentService,
 	ProvidePaymentOrderExpiryService,
+	ProvideBalanceNotifyService,
 )
 
 // ProvidePaymentConfigService wraps NewPaymentConfigService to accept the named
@@ -473,6 +474,11 @@ func ProvidePaymentConfigService(entClient *dbent.Client, settingRepo SettingRep
 	return NewPaymentConfigService(entClient, settingRepo, []byte(key))
 }
 
+// ProvideBalanceNotifyService creates BalanceNotifyService
+func ProvideBalanceNotifyService(emailService *EmailService, settingRepo SettingRepository) *BalanceNotifyService {
+	return NewBalanceNotifyService(emailService, settingRepo)
+}
+
 // ProvidePaymentOrderExpiryService creates and starts PaymentOrderExpiryService.
 func ProvidePaymentOrderExpiryService(paymentSvc *PaymentService) *PaymentOrderExpiryService {
 	svc := NewPaymentOrderExpiryService(paymentSvc, 60*time.Second)