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/services/nearby_search.py

@@ -10,6 +10,7 @@ results toward the user's direction of travel and to pick a shorter content
 Not implemented in this iteration — callers should not pass them yet.
 """
 
+import logging
 from dataclasses import dataclass
 
 from geoalchemy2 import Geometry
@@ -21,6 +22,8 @@ from app.models.enums import ContentLanguage, ContentLength
 from app.models.place import Place
 from app.models.place_content import PlaceContent
 
+logger = logging.getLogger("guidecity.nearby_search")
+
 
 @dataclass
 class NearbyResult:
@@ -44,6 +47,14 @@ def find_nearby(
     city_id: int | None = None
     if city_slug is not None:
         city_id = db.scalar(select(City.id).where(City.slug == city_slug))
+        if city_id is None:
+            logger.warning("city_slug=%r not found; searching across all cities", city_slug)
+
+    logger.debug(
+        "nearby search starting: lat=%.5f lon=%.5f city_slug=%s lang=%s length=%s "
+        "min_results=%d initial_radius_m=%d step_m=%d max_radius_m=%d",
+        lat, lon, city_slug, language, length, min_results, initial_radius_m, step_m, max_radius_m,
+    )
 
     origin = func.ST_GeogFromText(f"SRID=4326;POINT({lon} {lat})")
     distance_expr = func.ST_Distance(Place.location, origin)
@@ -75,8 +86,16 @@ def find_nearby(
     while True:
         stmt = base_stmt.where(func.ST_DWithin(Place.location, origin, radius))
         rows = db.execute(stmt).all()
+        logger.debug("radius=%dm -> %d place(s) found", radius, len(rows))
         if len(rows) >= min_results or radius >= max_radius_m:
             break
         radius += step_m
 
+    if len(rows) < min_results:
+        logger.warning(
+            "hit max_radius_m=%dm with only %d/%d place(s) found near lat=%.5f lon=%.5f",
+            max_radius_m, len(rows), min_results, lat, lon,
+        )
+    logger.info("nearby search done: radius=%dm count=%d", radius, len(rows))
+
     return NearbyResult(search_radius_m=radius, rows=rows)