Fix test helper to use valid container folder path

- Change folder path from /tmp/test-library to /app/uploads/test-library
- /app/uploads is mounted volume in docker-compose test container
- Matches TEST_UPLOAD_PATH environment variable
- Fixes library folder creation 400 error in integration tests

Related: Library folder validation changes

cmd/server/tests/test_helpers.go

@@ -298,7 +298,7 @@ func createTestMediaItemID(t *testing.T, ts *httptest.Server, token string) stri
 
 	// Add a folder to the library (required before adding media items)
 	folderReq := map[string]interface{}{
-		"folder_path": "/tmp/test-library",
+		"folder_path": "/app/uploads/test-library",
 	}
 	folderBody, _ := json.Marshal(folderReq)