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/bookings/service_test.go

@@ -3,6 +3,8 @@ package bookings
 import (
 	"context"
 	"errors"
+	"fmt"
+	"net/url"
 	"testing"
 	"time"
 
@@ -399,3 +401,52 @@ func TestCreateAppointmentRejectsForeignStaff(t *testing.T) {
 		t.Fatalf("expected ErrInvalidBooking, got %v", err)
 	}
 }
+
+func TestCustomerCancelPromotesWaitlist(t *testing.T) {
+	repo := db.NewMemoryRepository()
+	service := NewService(repo, nil, "http://localhost:3000")
+	customerService := NewCustomerService(repo, nil, "http://localhost:3000")
+	ctx := context.Background()
+
+	session, err := repo.GetClassSessionByID(ctx, "4bf74c12-44dd-45ca-86bb-b104f16f2435")
+	if err != nil {
+		t.Fatalf("get session: %v", err)
+	}
+	sessionID := session.ID
+	book := func(email string) domain.CreateBookingResponse {
+		resp, err := service.Create(ctx, domain.CreateBookingRequest{
+			TenantSlug:     "studio-atelier",
+			BookingMode:    "class",
+			ClassSessionID: &sessionID,
+			CustomerName:   "Test User",
+			CustomerEmail:  email,
+			StartsAt:       session.StartsAt.UTC().Format(time.RFC3339),
+			EndsAt:         session.EndsAt.UTC().Format(time.RFC3339),
+		})
+		if err != nil {
+			t.Fatalf("book %s: %v", email, err)
+		}
+		return resp
+	}
+
+	var confirmed []domain.CreateBookingResponse
+	for i := 0; i < 4; i++ {
+		confirmed = append(confirmed, book(fmt.Sprintf("seat%[email protected]", i)))
+	}
+	waiter := book("[email protected]")
+	if waiter.Status != "waitlisted" {
+		t.Fatalf("expected waitlisted, got %s", waiter.Status)
+	}
+
+	// Customer cancels a confirmed seat via the management token.
+	manageURL, _ := url.Parse(confirmed[0].ManageURL)
+	token := manageURL.Query().Get("token")
+	if err := customerService.CancelBooking(ctx, confirmed[0].Reference, token); err != nil {
+		t.Fatalf("customer cancel: %v", err)
+	}
+
+	booking, _ := repo.GetBookingByID(ctx, waiter.BookingID)
+	if booking.Status != "confirmed" {
+		t.Fatalf("expected waiter promoted to confirmed, got %q", booking.Status)
+	}
+}