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