feat(ECR-014): MembershipPlan 套餐配置并 Closed

membership_plans 表、admin 套餐页、Grant/CreateOrder 读表；
Loop continuous 自动 Approve/Closed。Next：ECR-015 RedemptionCode。

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/api/internal/service/admin/service.go

@@ -181,7 +181,7 @@ type GrantInput struct {
 
 // GrantMembership extends membership and writes audit.
 func (s *Service) GrantMembership(ctx context.Context, adminID, userID uuid.UUID, plan string) error {
-	days, err := planDays(plan)
+	days, err := s.PlanDurationDays(ctx, plan)
 	if err != nil {
 		return err
 	}
@@ -230,16 +230,7 @@ func (s *Service) ListAuditLogs(ctx context.Context, limit, offset int) ([]repos
 }
 
 func planDays(plan string) (int, error) {
-	switch plan {
-	case "month":
-		return 31, nil
-	case "quarter":
-		return 92, nil
-	case "year":
-		return 366, nil
-	default:
-		return 0, ErrInvalidPlan
-	}
+	return planDaysFallback(plan)
 }
 
 func newToken() (string, error) {