Fix FastAPI response model for geolocate endpoint

Union return type GeolocateOut | Response is not a valid Pydantic field
type. Use explicit response_model decorator parameter instead.

search/src/orrery_search/routers/observer.py

@@ -31,8 +31,8 @@ def _client_ip(request: Request) -> str | None:
     return request.client.host if request.client else None
 
 
-@router.get("/geolocate")
-def geolocate(request: Request) -> GeolocateOut | Response:
+@router.get("/geolocate", response_model=GeolocateOut, responses={204: {}})
+def geolocate(request: Request):
     """Return approximate location from client IP, or 204 if unavailable.
 
     Synchronous endpoint: geoip2 Reader.city() does mmap I/O which would