Add guests API with CRUD operations and tests

Introduce a new API for managing event guests, including endpoints for creating, reading, updating, deleting, and changing guest status. Added corresponding Pydantic schemas, database CRUD logic, and extensive test coverage to validate functionality. Integrated the guests API under the events router.

backend/app/api/routes/events/router.py

@@ -21,9 +21,12 @@ from app.schemas.events import (
     EventResponse, Event,
 )
 
+from app.api.routes.events import guests
+
 logger = logging.getLogger(__name__)
 events_router = APIRouter()
 
+events_router.include_router(guests.router, prefix="/guests", tags=["guests"])
 
 def validate_event_access(
         *,