feat(supervisor): allow up to 3 supervisors per owner

internal/quitcheckin/service/supervisor.go

@@ -40,14 +40,17 @@ type SupervisorStatusResult struct {
 }
 
 var (
-	ErrInviteNotFound  = errors.New("邀请不存在")
-	ErrInviteExpired   = errors.New("邀请已过期")
-	ErrInviteUsed      = errors.New("邀请已被使用")
-	ErrCannotBindSelf  = errors.New("不能绑定自己为监督人")
-	ErrBindingExists   = errors.New("监督关系已存在")
-	ErrBindingNotFound = errors.New("监督关系不存在")
+	ErrInviteNotFound         = errors.New("邀请不存在")
+	ErrInviteExpired          = errors.New("邀请已过期")
+	ErrInviteUsed             = errors.New("邀请已被使用")
+	ErrCannotBindSelf         = errors.New("不能绑定自己为监督人")
+	ErrBindingExists          = errors.New("监督关系已存在")
+	ErrBindingNotFound        = errors.New("监督关系不存在")
+	ErrSupervisorLimitReached = errors.New("监督人已达上限")
 )
 
+const maxSupervisorsPerOwner = 3
+
 func (s *Service) CreateSupervisorInvite(ctx context.Context, ownerUID int, now time.Time, days int) (SupervisorInviteResult, error) {
 	if days <= 0 {
 		days = 7
@@ -113,6 +116,18 @@ func (s *Service) BindSupervisorInvite(ctx context.Context, supervisorUID int, t
 			return err
 		}
 
+		// 多人监督：允许多个 supervisor，但限制最多 3 个 active supervisor。
+		var activeCount int64
+		if err := tx.WithContext(ctx).
+			Model(&quitmodel.SupervisorBinding{}).
+			Where("owner_uid = ? AND status = ?", invite.OwnerUID, "active").
+			Count(&activeCount).Error; err != nil {
+			return err
+		}
+		if activeCount >= maxSupervisorsPerOwner {
+			return ErrSupervisorLimitReached
+		}
+
 		binding := quitmodel.SupervisorBinding{
 			OwnerUID:      invite.OwnerUID,
 			SupervisorUID: supervisorUID,