Add structured logging throughout the backend

Request-level logging middleware (method/path/status/duration),
per-module loggers in routes/services (nearby search radius
expansion, 404s, name search), LOG_LEVEL setting wired through
docker-compose, and seed_loader converted from print() to logging.

Verified: pytest passes, and manually confirmed log lines appear for
both successful and 404 requests via `docker compose logs api`.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/api/routes/cities.py

@@ -1,3 +1,5 @@
+import logging
+
 from fastapi import APIRouter, Depends
 from geoalchemy2 import Geometry
 from sqlalchemy import cast, func, select
@@ -8,6 +10,8 @@ from app.models.city import City
 from app.schemas.city import CityOut
 from app.schemas.common import LatLon, LocalizedName
 
+logger = logging.getLogger("guidecity.cities")
+
 router = APIRouter(tags=["cities"])
 
 
@@ -31,4 +35,5 @@ def list_cities(db: Session = Depends(get_db)) -> list[CityOut]:
                 center=center,
             )
         )
+    logger.debug("listed %d cities", len(out))
     return out