fix(tests): Add missing addFolderToLibrary call to TestMediaItemISBNNormalization

- TestMediaItemISBNNormalization was missing library folder creation
- Caused HTTP 400 errors when creating media items
- addFolderToLibrary call was accidentally removed from line 73
- Tests now properly create library with folder before adding media items

This fixes the root cause of ISBN normalization test failures where
media-item creation failed due to missing library folder requirement.

Dockerfile

@@ -59,7 +59,7 @@ ENV RATE_LIMIT_ENABLED=false
 ENV REQUESTS_PER_MINUTE=1000
 
 # Default command for running tests
-CMD ["go", "test", "./cmd/server/tests", "-v", "-timeout", "5m"]
+CMD ["go", "test", "./cmd/server/tests", "-v", "-timeout", "5m", "-parallel=1"]
 
 # Final stage
 FROM alpine:latest