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/nearby.py

@@ -1,3 +1,5 @@
+import logging
+
 from fastapi import APIRouter, Depends, Query
 from sqlalchemy.orm import Session
 
@@ -8,6 +10,8 @@ from app.schemas.nearby import NearbyPlace, NearbyResponse
 from app.schemas.place import ContentOut
 from app.services.nearby_search import find_nearby
 
+logger = logging.getLogger("guidecity.nearby")
+
 router = APIRouter(tags=["nearby"])
 
 
@@ -32,6 +36,9 @@ def nearby(
     ),
     db: Session = Depends(get_db),
 ) -> NearbyResponse:
+    if speed is not None or heading is not None:
+        logger.debug("nearby called with reserved v2 params speed=%s heading=%s (ignored)", speed, heading)
+
     result = find_nearby(
         db,
         lat=lat,