feat(tier2): day-of check-in — Block H

QR codes on RSVP confirmations, a phone-friendly door scanner, walk-in
support, and a live arrivals widget that updates over WebSocket. Closes
the final Tier 2 block.

Schema (migration 0013)
- check_ins (id, guest_id UNIQUE, checked_in_at, checked_in_by,
  arrival_count, notes, walk_in). UNIQUE on guest_id is the
  double-check-in guard at the DB layer; signature validation lives
  in the QR JWT.

QR JWT
- internal/auth/checkin_qr.go: CheckInQRSigner mints {event_id,
  guest_id, exp} payloads with the platform's existing HMAC secret.
  Issue() extends expiry to eventDate+24h so a QR minted weeks in
  advance still scans on the day. Parse() distinguishes
  ErrExpiredJWT from generic ErrInvalidJWT so the API can render a
  friendlier 410.
- Unit tests cover round-trip, wrong-secret rejection, expiry
  detection, and short-secret refusal at construction time.

Domain + storage
- domain.CheckIn + CheckInSummary
- storage.CheckInRepo: Record (returns ErrAlreadyCheckedIn on the
  unique violation), ListByEvent, Summary (arrived headcount,
  expected headcount, guests-checked-in count), GuestBelongsToEvent
  (belt-and-braces guard against a forged JWT pointing at a
  different event's guest).

API
- GET /access/{token} now embeds a check_in payload (raw JWT + a
  base64-encoded PNG via skip2/go-qrcode) for attending RSVPs, so
  the confirmation page can render the code straight into an <img>.
- POST /events/{id}/check-in — editor+. Validates the QR JWT,
  refuses cross-event payloads (400), refuses expired ones (410),
  records the row, broadcasts check_in.recorded over the existing
  WS hub so the live dashboard updates.
- POST /events/{id}/walk-ins — editor+. Creates the guest + check-in
  in one logical op for a door-add who wasn't on the original list.
- GET /events/{id}/check-ins — viewer+. Returns the list and the
  summary together so the dashboard widget hydrates in one call.

Frontend
- New CheckInCard.vue: live arrivals widget ("47 of 60 · 78%" plus
  a progress bar), recent-arrivals list, Walk-in button, and a
  "Start scanning" button that opens a full-screen camera modal.
  jsQR loaded from CDN on first open (no bundler dep). Scan
  throttling + dedupe prevents the 30fps camera loop from POSTing
  N times per paper QR. Successful scan vibrates the phone.
  Duplicate (409) → "Already checked in" toast; expired (410) →
  "This code has expired"; foreign-event (400) → "doesn't look
  like one of your guests".
- New "Check-in" tab on the event-detail page, between
  Communications and Branding.
- RSVP confirmation card + revisit card both surface a "Save for
  the day" / "Your door code" QR block for attending guests. The
  PNG ships pre-rendered from the API so the frontend doesn't need
  its own QR library.
- The submit flow now refetches /access after a successful POST so
  the QR appears immediately on first submit, not just on revisit.

Tests
- Backend unit tests for the QR signer (round-trip, wrong-secret,
  expired, short-secret rejection).
- Integration: TestCheckInHappyPath (scan -> 200, double-scan ->
  409, summary reflects arrival), TestCheckInRejectsForeignQR
  (event A's JWT can't be used on event B), TestWalkInCreatesGuest
  AndCheckIn (door-add creates both rows).
- Full integration suite passes (188.3s, 41 tests / 80+ subtests).

Tier 2 is complete: Blocks A through H all shipped.

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

frontend/pages/dashboard/events/[id].vue

@@ -69,8 +69,8 @@ const loading = ref(true)
 // (Collaborators + Branding, configured once) → Analytics (results,
 // checked periodically). The two action-y tabs anchor the ends; setup
 // clusters in the middle.
-type EventTab = 'guests' | 'collaborators' | 'communications' | 'branding' | 'analytics' | 'gate'
-const validTabs: EventTab[] = ['guests', 'collaborators', 'communications', 'branding', 'analytics', 'gate']
+type EventTab = 'guests' | 'collaborators' | 'communications' | 'branding' | 'analytics' | 'gate' | 'checkin'
+const validTabs: EventTab[] = ['guests', 'collaborators', 'communications', 'branding', 'analytics', 'gate', 'checkin']
 function tabFromHash(): EventTab {
   if (import.meta.client) {
     const h = window.location.hash.replace('#', '') as EventTab
@@ -786,6 +786,7 @@ function checkLabel(band?: string): string {
           { id: 'guests',         label: 'Guests' },
           { id: 'collaborators',  label: 'Collaborators' },
           { id: 'communications', label: 'Communications' },
+          { id: 'checkin',        label: 'Check-in' },
           { id: 'branding',       label: 'Branding' },
           { id: 'analytics',      label: 'Analytics' },
           { id: 'gate',           label: 'Gate' },
@@ -1218,6 +1219,11 @@ function checkLabel(band?: string): string {
       <CommunicationsCard :event-id="eventId" :your-role="event.your_role" />
     </div>
 
+    <!-- Check-in (Tier 2 Block H). Door scanner + live arrivals. -->
+    <div v-if="activeTab === 'checkin' && event" class="mt-2">
+      <CheckInCard :event-id="eventId" :your-role="event.your_role" />
+    </div>
+
     <!-- Gate (Tier 2 Block G). The user-facing rebrand of the fraud
          detector: strictness presets + trusted networks + decision
          history, with the technical sliders/CIDR jargon tucked behind