refactor(storage): move file management routes from user to admin namespace

- Remove file list, stats, download, and deletion routes from '/api/v1/upload'
- Move these endpoints under '/api/v1/admin/uploads'
- Remove user-specific filtering from files query and statistics to aggregate system-wide uploads by default
- Allow admins to bypass ownership check when downloading private files
- Update backend unit tests, Swagger documentation, and frontend service client and components

internal/apps/upload/file_server.go

@@ -292,6 +292,9 @@ func checkPrivateFileOwner(c *gin.Context, ownerID uint64) error {
 			return err
 		}
 	}
+	if currUser.IsAdmin {
+		return nil
+	}
 	if currUser.ID != ownerID {
 		return errors.New("forbidden: cross-user access denied")
 	}