feat: add shared media assets and fix credit lifecycle

credits.go

@@ -33,7 +33,7 @@ func (a *App) reserveCredit(userID, taskID uint, amount int64) (*CreditReservati
 			}
 		}
 		var existing CreditReservation
-		if err := tx.Where("task_id = ?", taskID).First(&existing).Error; err == nil {
+		if err := tx.Where("task_id = ? AND status = ?", taskID, "reserved").Order("id desc").First(&existing).Error; err == nil {
 			reservation = existing
 			return nil
 		} else if !errors.Is(err, gorm.ErrRecordNotFound) {
@@ -106,7 +106,9 @@ func (a *App) refundCredit(task *Task) error {
 			return nil
 		}
 		if r.Status == "committed" {
-			return errReservationState
+			// A committed reservation belongs to a completed download. A later
+			// storage/retry error must never debit or refund it a second time.
+			return nil
 		}
 		if r.Status != "reserved" {
 			return errReservationState