Add storage utilities and tests for file handling and tokens

Introduced new fixtures and tests for storage functionality, including saving files, generating URLs, and token creation/verification. Refactored `get_storage_provider` into a separate dependency module. Enhanced test coverage for improved reliability.

backend/app/api/dependencies/common.py

@@ -0,0 +1,10 @@
+from app.core.config import settings
+from app.core.storage import StorageProvider, LocalStorageProvider
+
+
+def get_storage_provider() -> StorageProvider:
+    """Dependency for getting the configured storage provider."""
+    return LocalStorageProvider(
+        upload_folder=settings.UPLOAD_FOLDER,
+        files_url_path="/files"
+    )