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/handler/dto/mappers.go

@@ -13,16 +13,19 @@ func UserFromServiceShallow(u *service.User) *User {
 		return nil
 	}
 	return &User{
-		ID:            u.ID,
-		Email:         u.Email,
-		Username:      u.Username,
-		Role:          u.Role,
-		Balance:       u.Balance,
-		Concurrency:   u.Concurrency,
-		Status:        u.Status,
-		AllowedGroups: u.AllowedGroups,
-		CreatedAt:     u.CreatedAt,
-		UpdatedAt:     u.UpdatedAt,
+		ID:                       u.ID,
+		Email:                    u.Email,
+		Username:                 u.Username,
+		Role:                     u.Role,
+		Balance:                  u.Balance,
+		Concurrency:              u.Concurrency,
+		Status:                   u.Status,
+		AllowedGroups:            u.AllowedGroups,
+		CreatedAt:                u.CreatedAt,
+		UpdatedAt:                u.UpdatedAt,
+		BalanceNotifyEnabled:     u.BalanceNotifyEnabled,
+		BalanceNotifyThreshold:   u.BalanceNotifyThreshold,
+		BalanceNotifyExtraEmails: u.BalanceNotifyExtraEmails,
 	}
 }
 
@@ -322,6 +325,26 @@ func AccountFromServiceShallow(a *service.Account) *Account {
 				out.QuotaWeeklyResetAt = &v
 			}
 		}
+
+		// 配额通知配置
+		if enabled := a.GetQuotaNotifyDailyEnabled(); enabled {
+			out.QuotaNotifyDailyEnabled = &enabled
+		}
+		if threshold := a.GetQuotaNotifyDailyThreshold(); threshold > 0 {
+			out.QuotaNotifyDailyThreshold = &threshold
+		}
+		if enabled := a.GetQuotaNotifyWeeklyEnabled(); enabled {
+			out.QuotaNotifyWeeklyEnabled = &enabled
+		}
+		if threshold := a.GetQuotaNotifyWeeklyThreshold(); threshold > 0 {
+			out.QuotaNotifyWeeklyThreshold = &threshold
+		}
+		if enabled := a.GetQuotaNotifyTotalEnabled(); enabled {
+			out.QuotaNotifyTotalEnabled = &enabled
+		}
+		if threshold := a.GetQuotaNotifyTotalThreshold(); threshold > 0 {
+			out.QuotaNotifyTotalThreshold = &threshold
+		}
 	}
 
 	return out

backend/internal/handler/dto/settings.go

@@ -148,6 +148,11 @@ type SystemSettings struct {
 	PaymentCancelRateLimitWindow  int    `json:"payment_cancel_rate_limit_window"`
 	PaymentCancelRateLimitUnit    string `json:"payment_cancel_rate_limit_unit"`
 	PaymentCancelRateLimitMode    string `json:"payment_cancel_rate_limit_window_mode"`
+
+	// Balance low notification
+	BalanceLowNotifyEnabled   bool     `json:"balance_low_notify_enabled"`
+	BalanceLowNotifyThreshold float64  `json:"balance_low_notify_threshold"`
+	AccountQuotaNotifyEmails  []string `json:"account_quota_notify_emails"`
 }
 
 type DefaultSubscriptionSetting struct {

backend/internal/handler/dto/types.go

@@ -18,6 +18,11 @@ type User struct {
 	CreatedAt     time.Time `json:"created_at"`
 	UpdatedAt     time.Time `json:"updated_at"`
 
+	// 余额不足通知
+	BalanceNotifyEnabled     bool     `json:"balance_notify_enabled"`
+	BalanceNotifyThreshold   *float64 `json:"balance_notify_threshold"`
+	BalanceNotifyExtraEmails []string `json:"balance_notify_extra_emails"`
+
 	APIKeys       []APIKey           `json:"api_keys,omitempty"`
 	Subscriptions []UserSubscription `json:"subscriptions,omitempty"`
 }
@@ -218,6 +223,14 @@ type Account struct {
 	QuotaDailyResetAt    *string `json:"quota_daily_reset_at,omitempty"`
 	QuotaWeeklyResetAt   *string `json:"quota_weekly_reset_at,omitempty"`
 
+	// 配额通知配置
+	QuotaNotifyDailyEnabled    *bool    `json:"quota_notify_daily_enabled,omitempty"`
+	QuotaNotifyDailyThreshold  *float64 `json:"quota_notify_daily_threshold,omitempty"`
+	QuotaNotifyWeeklyEnabled   *bool    `json:"quota_notify_weekly_enabled,omitempty"`
+	QuotaNotifyWeeklyThreshold *float64 `json:"quota_notify_weekly_threshold,omitempty"`
+	QuotaNotifyTotalEnabled    *bool    `json:"quota_notify_total_enabled,omitempty"`
+	QuotaNotifyTotalThreshold  *float64 `json:"quota_notify_total_threshold,omitempty"`
+
 	Proxy         *Proxy         `json:"proxy,omitempty"`
 	AccountGroups []AccountGroup `json:"account_groups,omitempty"`