Add lightweight JSON WebSocket protocol (?mode=json) that bridges
Automerge to JSON for demo pages, avoiding the ~500KB Automerge bundle.
Includes GET /api/communities/:slug/shapes endpoint, POST demo reset
with rate limiting, Alpine Explorer 2026 seed data (~40 shapes), and
the useDemoSync React hook for real-time demo page connectivity.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
