feat(tier2): editable RSVPs — Block A

Guests can revisit their invitation link and change their response
or plus-ones up to 5 times. Each prior state is snapshotted into
`rsvp_revisions` and surfaced to the host via a per-guest history
modal on the event detail page.

- Migration 0007 adds rsvp_revisions + rsvps.edit_count (with down)
- RSVPRepo.Update wraps snapshot+update+counter in one transaction,
  FOR UPDATE-locking the row so concurrent edits can't bypass the cap
- PATCH /rsvp/{token} re-runs the fraud check on every edit attempt
  (different device on an edit is itself a signal)
- POST /rsvp no longer marks the token used — the link stays valid
  so the guest can come back to edit
- GET /access/{token} now embeds the existing RSVP so the frontend
  renders an edit form instead of a blank submit form on revisit
- New host endpoint GET /events/{id}/guests/{guest_id}/rsvp/history
- Frontend: rsvp/[token].vue toggles between summary + edit form,
  surfaces edits-remaining; dashboard adds a "History" action on
  responded guests opening a revision-trail modal

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/storage/migrations/0007_rsvp_edits.down.sql

@@ -0,0 +1,4 @@
+ALTER TABLE rsvps DROP COLUMN IF EXISTS edit_count;
+
+DROP INDEX IF EXISTS idx_rsvp_revisions_rsvp;
+DROP TABLE IF EXISTS rsvp_revisions;