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/core/logging.py

@@ -1,4 +1,11 @@
 import logging
+import time
+from collections.abc import Awaitable, Callable
+
+from starlette.requests import Request
+from starlette.responses import Response
+
+request_logger = logging.getLogger("guidecity.http")
 
 
 def configure_logging(level: int = logging.INFO) -> None:
@@ -6,3 +13,25 @@ def configure_logging(level: int = logging.INFO) -> None:
         level=level,
         format="%(asctime)s %(levelname)s %(name)s %(message)s",
     )
+
+
+async def log_requests(
+    request: Request,
+    call_next: Callable[[Request], Awaitable[Response]],
+) -> Response:
+    """Logs method/path/status/duration for every request handled by the app."""
+    start = time.perf_counter()
+    try:
+        response = await call_next(request)
+    except Exception:
+        duration_ms = (time.perf_counter() - start) * 1000
+        request_logger.exception(
+            "%s %s -> unhandled exception (%.1fms)", request.method, request.url.path, duration_ms
+        )
+        raise
+
+    duration_ms = (time.perf_counter() - start) * 1000
+    request_logger.info(
+        "%s %s -> %d (%.1fms)", request.method, request.url.path, response.status_code, duration_ms
+    )
+    return response