feat(notify): add percentage threshold type for balance low notification

- Add threshold_type field (fixed/percentage) to system and user settings
- Add total_recharged field to users table, auto-incremented on balance credit
- Percentage mode: effective threshold = total_recharged × percentage / 100
- User-level threshold_type inherits from system default when not set
- Update admin settings UI with radio selector (fixed amount / percentage)
- Migration: 102_add_balance_notify_threshold_type.sql

backend/internal/service/user_service.go

@@ -62,11 +62,12 @@ type UserRepository interface {
 
 // UpdateProfileRequest 更新用户资料请求
 type UpdateProfileRequest struct {
-	Email                  *string  `json:"email"`
-	Username               *string  `json:"username"`
-	Concurrency            *int     `json:"concurrency"`
-	BalanceNotifyEnabled   *bool    `json:"balance_notify_enabled"`
-	BalanceNotifyThreshold *float64 `json:"balance_notify_threshold"`
+	Email                      *string  `json:"email"`
+	Username                   *string  `json:"username"`
+	Concurrency                *int     `json:"concurrency"`
+	BalanceNotifyEnabled       *bool    `json:"balance_notify_enabled"`
+	BalanceNotifyThresholdType *string  `json:"balance_notify_threshold_type"`
+	BalanceNotifyThreshold     *float64 `json:"balance_notify_threshold"`
 }
 
 // ChangePasswordRequest 修改密码请求
@@ -143,6 +144,9 @@ func (s *UserService) UpdateProfile(ctx context.Context, userID int64, req Updat
 	if req.BalanceNotifyEnabled != nil {
 		user.BalanceNotifyEnabled = *req.BalanceNotifyEnabled
 	}
+	if req.BalanceNotifyThresholdType != nil {
+		user.BalanceNotifyThresholdType = *req.BalanceNotifyThresholdType
+	}
 	if req.BalanceNotifyThreshold != nil {
 		if *req.BalanceNotifyThreshold <= 0 {
 			user.BalanceNotifyThreshold = nil // clear to system default