Add folder validation to CreateMediaItem handler

- Check library has folders before creating media items
- Return HTTP 400 with clear error message if no folders
- Proper error code (400) instead of generic 500
- Improved user feedback for invalid operations
- Inject LibraryService into MediaHandler

Fixes: TestCollectionsBulkOperations HTTP 500 errors
Related: Service layer validation commit

cmd/server/main.go

@@ -92,6 +92,9 @@ func main() {
 	// Create sync queue processor
 	queueProcessor := sync.NewSyncQueueProcessor(queries)
 
+	// Create library service
+	libraryService := services.NewLibraryService(queries)
+
 	// Create worker for background tasks
 	worker := services.NewWorker(3)
 
@@ -107,7 +110,7 @@ func main() {
 
 	// NEW: Create refactored handlers
 	collectionHandler := handlers.NewCollectionHandler(queries, connManager)
-	mediaHandler := handlers.NewMediaHandler(queries, worker)
+	mediaHandler := handlers.NewMediaHandler(queries, libraryService, worker)
 	searchHandler := handlers.NewSearchHandler(queries)
 	matchingHandler := handlers.NewMatchingHandler(queries, connManager)