Rename schema and test files for consistency

Renamed `event_theme` and `test_user_schema` file paths and imports to follow consistent plural naming conventions. This improves code clarity and aligns file and import naming across the project.

backend/app/schemas/event_themes.py

@@ -1,4 +1,4 @@
-# app/schemas/event_theme.py
+# app/schemas/event_themes.py
 from typing import Dict, Optional
 from uuid import UUID  # Add this import
 from pydantic import BaseModel, Field, validator, field_validator