refactor: standardize API response formats for list endpoints

- ListLibraries now returns {"data": []} instead of []
- ListUsers now returns {"data": []} instead of []
- ListMediaItems now returns {"data": []} instead of []

This provides consistent response structure across all list endpoints
and aligns with integration test expectations.

internal/handlers/auth.go

@@ -554,7 +554,7 @@ func (h *AuthHandler) ListUsers(c echo.Context) error {
 		})
 	}
 
-	return c.JSON(http.StatusOK, userList)
+	return c.JSON(http.StatusOK, map[string]interface{}{"data": userList})
 }
 
 type AddEbookFolderRequest struct {