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/tests/schemas/test_event_themes.py

@@ -1,7 +1,7 @@
 import pytest
 from uuid import UUID
 from pydantic import ValidationError
-from app.schemas.event_theme import (
+from app.schemas.event_themes import (
     EventThemeCreate,
     EventThemeUpdate,
     EventThemeResponse,

backend/tests/schemas/test_users.py

@@ -1,4 +1,4 @@
-# tests/schemas/test_user_schema.py
+# tests/schemas/test_users.py
 import pytest
 import re
 from pydantic import ValidationError