- Add POST /api/v1/check-in endpoint with CheckInSubmission
- nearby_cafes returns HTML fragment for Datastar, JSON for API
- Add NearbyCafeView and NearbyCafesFragment for server-rendered results
- Rewrite checkin.html: replace checkin.js with Datastar signals,
@post for submission, @get for nearby search, inline geolocation JS
- Cafe filtering uses data-show with signal-based search
