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>

internal/auth/checkin_qr_test.go

@@ -0,0 +1,76 @@
+package auth
+
+import (
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+const qrTestSecret = "test-secret-must-be-at-least-32-bytes-long-xx"
+
+func TestCheckInQR_RoundTrip(t *testing.T) {
+	s, err := NewCheckInQRSigner(qrTestSecret, "test", 6*time.Hour)
+	if err != nil {
+		t.Fatalf("new signer: %v", err)
+	}
+	eventID := uuid.New()
+	guestID := uuid.New()
+	now := time.Now().UTC()
+	tok, exp, err := s.Issue(eventID, guestID, now.Add(24*time.Hour), now)
+	if err != nil {
+		t.Fatalf("issue: %v", err)
+	}
+	if !exp.After(now) {
+		t.Fatalf("expiry should be in the future: %v vs now %v", exp, now)
+	}
+
+	claims, err := s.Parse(tok)
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if claims.EventID != eventID || claims.GuestID != guestID {
+		t.Errorf("ids mismatch: got %v / %v, want %v / %v",
+			claims.EventID, claims.GuestID, eventID, guestID)
+	}
+}
+
+func TestCheckInQR_RejectsWrongSecret(t *testing.T) {
+	signerA, _ := NewCheckInQRSigner(qrTestSecret, "test", time.Hour)
+	signerB, _ := NewCheckInQRSigner("other-secret-must-be-at-least-32-bytes-xx", "test", time.Hour)
+
+	tok, _, err := signerA.Issue(uuid.New(), uuid.New(), time.Now().Add(time.Hour), time.Now())
+	if err != nil {
+		t.Fatalf("issue: %v", err)
+	}
+	if _, err := signerB.Parse(tok); !errors.Is(err, ErrInvalidJWT) {
+		t.Errorf("parse with wrong secret: want ErrInvalidJWT, got %v", err)
+	}
+}
+
+func TestCheckInQR_RejectsExpired(t *testing.T) {
+	s, _ := NewCheckInQRSigner(qrTestSecret, "test", time.Second)
+	// Use a "now" far enough in the past that even the eventDate+24h
+	// extension lands before the actual current time. Two days ago for
+	// both: expiry resolves to now-2d+1s OR now-2d+24h, the later wins,
+	// so the token expires at now-1d — still in the past.
+	pastNow := time.Now().UTC().Add(-48 * time.Hour)
+	eventDate := pastNow
+	tok, exp, err := s.Issue(uuid.New(), uuid.New(), eventDate, pastNow)
+	if err != nil {
+		t.Fatalf("issue: %v", err)
+	}
+	if exp.After(time.Now().UTC()) {
+		t.Fatalf("setup bug: expected exp %v in the past", exp)
+	}
+	if _, err := s.Parse(tok); !errors.Is(err, ErrExpiredJWT) {
+		t.Errorf("parse expired: want ErrExpiredJWT, got %v", err)
+	}
+}
+
+func TestCheckInQR_SecretTooShort(t *testing.T) {
+	if _, err := NewCheckInQRSigner("short", "test", time.Hour); err == nil {
+		t.Error("expected error for too-short secret")
+	}
+}