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/places.py

@@ -1,3 +1,5 @@
+import logging
+
 from fastapi import APIRouter, Depends, HTTPException, Query
 from geoalchemy2 import Geometry
 from sqlalchemy import and_, cast, func, select
@@ -13,6 +15,8 @@ from app.schemas.common import LatLon
 from app.schemas.place import ContentOut, PlaceDetail, PlaceListItem
 from app.services.geocoding import search_places
 
+logger = logging.getLogger("guidecity.places")
+
 router = APIRouter(tags=["places"])
 
 
@@ -56,6 +60,7 @@ def get_place(
     )
     row = db.execute(stmt).first()
     if row is None:
+        logger.warning("place_id=%d not found for lang=%s length=%s", place_id, lang, length)
         raise HTTPException(status_code=404, detail="Place not found")
 
     place, content, lon, lat = row
@@ -84,6 +89,7 @@ def list_city_places(
 ) -> list[PlaceListItem]:
     city = db.scalar(select(City).where(City.slug == city_slug))
     if city is None:
+        logger.warning("city_slug=%r not found", city_slug)
         raise HTTPException(status_code=404, detail="City not found")
 
     stmt = (
@@ -111,6 +117,7 @@ def list_city_places(
         stmt = stmt.where(PlaceContent.title.ilike(f"%{q}%"))
 
     rows = db.execute(stmt).all()
+    logger.debug("city_slug=%r listing -> %d place(s)", city_slug, len(rows))
     return [
         PlaceListItem(id=p.id, slug=p.slug, category=p.category, name=pc.title, location=LatLon(lat=lat, lon=lon))
         for p, pc, lon, lat in rows