Additional Response Headers

Either to the Response Headers or Response data, it would be helpful to include `last-modified` - a date for when the item, monster, etc. was added to the SRC or last edited. This would help identify when an individual index ( i.e animated-shield ) was last modified.

Currently `date` Response Header outputs today's date.

Look at the API response handling code to see where headers are set. The issue mentions the 'date' header currently outputs today's date; find that logic and modify it to include a 'last-modified' header based on the item's actual modification date. Check the data models for monsters and items to see if they have timestamps. Test by making a request to an endpoint like /api/monsters or /api/items and verifying the new header appears.

索引モデルが issue の本文から書いたものです。