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/config.py

@@ -6,6 +6,7 @@ class Settings(BaseSettings):
 
     database_url: str = "postgresql+psycopg://guidecity:guidecity@localhost:5432/guidecity"
     api_v1_prefix: str = "/api/v1"
+    log_level: str = "INFO"
 
 
 settings = Settings()