[BACKEND] RSVP system — event_rsvps D1 table + POST/DELETE /events/:id/rsvp

[linear]

The frontend RSVP flow is complete (event detail screen, ConfirmModal, RsvpSuccessToast) but currently uses a temporary in-memory store (app/lib/rsvpStore.ts). This ticket replaces that with real backend support.

Build steps

1. Create event_rsvps table in D1:

CREATE TABLE event_rsvps (
  user_id TEXT NOT NULL,
  event_id INTEGER NOT NULL,
  created_at TEXT NOT NULL DEFAULT (datetime('now')),
  PRIMARY KEY (user_id, event_id)
);

2. POST /events/:id/rsvp — auth-gated, inserts a row (idempotent)
3. DELETE /events/:id/rsvp — auth-gated, removes row
4. Expose is_rsvped: boolean on GET /events/:id response (join against event_rsvps using the authed user's ID)
5. Update app/lib/rsvpStore.ts callers to use the real API once endpoints are live

Notes

• Caller signatures in rsvpStore.ts are already designed to match the future API — function names and Promise return types should not need to change
• Gate all writes on a valid JWT; unauthenticated requests → 401

[linear]

LOOP-220