Add event themes API routes for CRUD operations

Implemented API endpoints for creating, reading, updating, and listing event themes. Integrated the new routes into the FastAPI application router under the '/event_themes' prefix.

backend/app/api/main.py

@@ -1,6 +1,7 @@
 from fastapi import APIRouter
 
 from app.api.routes import auth
-
+from app.api.routes import event_themes
 api_router = APIRouter()
 api_router.include_router(auth.router, prefix="/auth", tags=["auth"])
+api_router.include_router(event_themes.router, prefix="/event_themes", tags=["event_themes"])