feat(bookings): dashboard cancellation and waitlist promotion

- POST /v1/catalog/bookings/{id}/cancel lets businesses cancel
  tenant-owned bookings; two-step confirm in the detail modal
- Shared PromoteWaitlist helper promotes earliest waitlisted bookings
  when a class seat frees, on both dashboard and customer cancel paths;
  promoted customers get a confirmation email, waitlist ledger marked
  resolved
- waitlisted status surfaced in bookings UI: stats card, filter, warning
  badge, demo seed, CS+EN strings
- BookingRecord gains created_at for fair ordering; GetBookingByID and
  ResolveWaitlistEntry added to the repository surface

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/catalog/service_test.go

@@ -3,6 +3,7 @@ package catalog
 import (
 	"context"
 	"errors"
+	"fmt"
 	"testing"
 	"time"
 
@@ -547,3 +548,95 @@ func TestDeleteClassSessionPreservesBooking(t *testing.T) {
 		t.Fatal("expected booking class_session_id to be nulled")
 	}
 }
+
+const demoSessionID = "4bf74c12-44dd-45ca-86bb-b104f16f2435"
+
+func bookClass(t *testing.T, repo db.Repository, email string) domain.CreateBookingResponse {
+	t.Helper()
+	session, err := repo.GetClassSessionByID(context.Background(), demoSessionID)
+	if err != nil {
+		t.Fatalf("get session: %v", err)
+	}
+	bookingSvc := bookings.NewService(repo, nil, "")
+	resp, err := bookingSvc.Create(context.Background(), domain.CreateBookingRequest{
+		TenantSlug:     "studio-atelier",
+		CustomerName:   "Test User",
+		CustomerEmail:  email,
+		BookingMode:    "class",
+		ClassSessionID: &[]string{demoSessionID}[0],
+		StartsAt:       session.StartsAt.UTC().Format(time.RFC3339),
+		EndsAt:         session.EndsAt.UTC().Format(time.RFC3339),
+	})
+	if err != nil {
+		t.Fatalf("book class for %s: %v", email, err)
+	}
+	return resp
+}
+
+func TestCancelBookingLifecycle(t *testing.T) {
+	repo := db.NewMemoryRepository()
+	svc := newCatalogService(repo)
+	ctx := context.Background()
+
+	resp := bookClass(t, repo, "[email protected]")
+
+	if err := svc.CancelBooking(ctx, owner, resp.BookingID); err != nil {
+		t.Fatalf("cancel booking: %v", err)
+	}
+	booking, _ := repo.GetBookingByID(ctx, resp.BookingID)
+	if booking.Status != "cancelled" {
+		t.Fatalf("expected cancelled, got %q", booking.Status)
+	}
+
+	if err := svc.CancelBooking(ctx, owner, resp.BookingID); !errors.Is(err, ErrInvalidInput) {
+		t.Fatalf("expected ErrInvalidInput on second cancel, got %v", err)
+	}
+	if err := svc.CancelBooking(ctx, owner, "00000000-0000-0000-0000-000000000099"); !errors.Is(err, ErrBookingNotFound) {
+		t.Fatalf("expected ErrBookingNotFound, got %v", err)
+	}
+	stranger := domain.Principal{Subject: "someone-else", Role: "staff"}
+	if err := svc.CancelBooking(ctx, stranger, resp.BookingID); !errors.Is(err, ErrTenantMembership) {
+		t.Fatalf("expected ErrTenantMembership, got %v", err)
+	}
+}
+
+func TestCancelBookingPromotesWaitlist(t *testing.T) {
+	repo := db.NewMemoryRepository()
+	svc := newCatalogService(repo)
+	ctx := context.Background()
+
+	// Seeded session capacity is 4 — fill it, then overflow onto the waitlist.
+	var confirmed []domain.CreateBookingResponse
+	for i := 0; i < 4; i++ {
+		confirmed = append(confirmed, bookClass(t, repo, fmt.Sprintf("seat%[email protected]", i)))
+	}
+	first := bookClass(t, repo, "[email protected]")
+	second := bookClass(t, repo, "[email protected]")
+
+	booking, _ := repo.GetBookingByID(ctx, first.BookingID)
+	if booking.Status != "waitlisted" {
+		t.Fatalf("expected waitlisted, got %q", booking.Status)
+	}
+
+	// Cancel a confirmed seat -> earliest waiter is promoted, second stays queued.
+	if err := svc.CancelBooking(ctx, owner, confirmed[0].BookingID); err != nil {
+		t.Fatalf("cancel: %v", err)
+	}
+	booking, _ = repo.GetBookingByID(ctx, first.BookingID)
+	if booking.Status != "confirmed" {
+		t.Fatalf("expected first waiter promoted, got %q", booking.Status)
+	}
+	booking, _ = repo.GetBookingByID(ctx, second.BookingID)
+	if booking.Status != "waitlisted" {
+		t.Fatalf("expected second waiter still queued, got %q", booking.Status)
+	}
+
+	// Free another seat -> second waiter promoted.
+	if err := svc.CancelBooking(ctx, owner, confirmed[1].BookingID); err != nil {
+		t.Fatalf("cancel second: %v", err)
+	}
+	booking, _ = repo.GetBookingByID(ctx, second.BookingID)
+	if booking.Status != "confirmed" {
+		t.Fatalf("expected second waiter promoted, got %q", booking.Status)
+	}
+}