Add structured logging config

core/logging.py is referenced as a stub for structured logging setup (configure_logging()). Flesh out real structured logging (e.g. JSON logs, log levels via settings).

Start by reading core/logging.py to understand the current stub. Examine the project's settings to see how log levels are configured. Research Python's logging module and libraries like python-json-logger for structured JSON output. Implement configure_logging() to integrate these, ensuring logs are properly formatted and levels are controlled via settings. Test by running the service and verifying log output.

索引モデルが issue の本文から書いたものです。