feat(api): add Kittens API for daily cat images
Implements three endpoints for managing daily kitten images:
- GET /api/v1/kittens/daily: Returns today's kitten or 404 if none
- PUT /api/v1/kittens/daily: Ensures kitten exists, fetches if needed
- DELETE /api/v1/kittens/daily: Removes today's kitten (requires auth)
PUT and DELETE require X-API-Key authorization for non-localhost.