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/server/routes/user.go

@@ -26,6 +26,14 @@ func RegisterUserRoutes(
 			user.PUT("/password", h.User.ChangePassword)
 			user.PUT("", h.User.UpdateProfile)
 
+			// 通知邮箱管理
+			notifyEmail := user.Group("/notify-email")
+			{
+				notifyEmail.POST("/send-code", h.User.SendNotifyEmailCode)
+				notifyEmail.POST("/verify", h.User.VerifyNotifyEmail)
+				notifyEmail.DELETE("", h.User.RemoveNotifyEmail)
+			}
+
 			// TOTP 双因素认证
 			totp := user.Group("/totp")
 			{