Add event routes and enhance routing structure

Introduced routes for event management, including CRUD operations and querying by user or public visibility. Updated event themes routes with operation IDs for better documentation. Refactored `api/main.py` to `api/router.py` and integrated events routing into the API.

backend/app/api/router.py

@@ -0,0 +1,10 @@
+from fastapi import APIRouter
+
+from app.api.routes import auth
+from app.api.routes import event_themes
+from app.api.routes.events import router as events
+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"])
+
+api_router.include_router(events.events_router, prefix="/events", tags=["events"])