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/user.go

@@ -30,6 +30,11 @@ type User struct {
 	TotpEnabled         bool       // 是否启用 TOTP
 	TotpEnabledAt       *time.Time // TOTP 启用时间
 
+	// 余额不足通知
+	BalanceNotifyEnabled     bool
+	BalanceNotifyThreshold   *float64
+	BalanceNotifyExtraEmails []string
+
 	APIKeys       []APIKey
 	Subscriptions []UserSubscription
 }