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/customer_service.go

@@ -26,6 +26,7 @@ type CustomerService struct {
 }
 
 type CustomerNotifier interface {
+	SendBookingConfirmation(ctx context.Context, data notifications.BookingEmailData) error
 	SendBookingReschedule(ctx context.Context, data notifications.BookingEmailData) error
 	SendBookingCancellation(ctx context.Context, data notifications.BookingEmailData) error
 }
@@ -42,6 +43,10 @@ func NewCustomerService(repo db.Repository, notifier CustomerNotifier, frontendU
 
 type customerNoopNotifier struct{}
 
+func (n *customerNoopNotifier) SendBookingConfirmation(ctx context.Context, data notifications.BookingEmailData) error {
+	return nil
+}
+
 func (n *customerNoopNotifier) SendBookingReschedule(ctx context.Context, data notifications.BookingEmailData) error {
 	return nil
 }
@@ -194,6 +199,15 @@ func (s *CustomerService) CancelBooking(ctx context.Context, reference string, t
 	// Send cancellation email (async)
 	go s.sendCancellationEmail(booking)
 
+	// A freed class seat promotes the earliest waitlisted booking.
+	if booking.ClassSessionID != nil {
+		if promoted, err := PromoteWaitlist(ctx, s.repo, booking.TenantID, *booking.ClassSessionID); err == nil {
+			for _, promotedBooking := range promoted {
+				go s.sendPromotionEmail(promotedBooking)
+			}
+		}
+	}
+
 	return nil
 }
 
@@ -270,3 +284,37 @@ func (s *CustomerService) sendCancellationEmail(booking db.BookingRecord) {
 
 	s.notifier.SendBookingCancellation(ctx, emailData)
 }
+
+// sendPromotionEmail confirms a booking that moved off the class waitlist.
+func (s *CustomerService) sendPromotionEmail(booking db.BookingRecord) {
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	tenant, err := s.repo.GetTenantByID(ctx, booking.TenantID)
+	if err != nil {
+		return
+	}
+	brand, _ := s.repo.GetBrandProfile(ctx, tenant.ID)
+	brandColor := brand.PrimaryColor
+	if brandColor == "" {
+		brandColor = "#a65c3e"
+	}
+	serviceName, locationName := s.descriptors(ctx, booking)
+
+	s.notifier.SendBookingConfirmation(ctx, notifications.BookingEmailData{
+		TenantID:      tenant.ID,
+		TenantName:    tenant.Name,
+		TenantSlug:    tenant.Slug,
+		BrandColor:    brandColor,
+		CustomerName:  booking.CustomerName,
+		CustomerEmail: booking.CustomerEmail,
+		Service:       serviceName,
+		Location:      locationName,
+		Reference:     booking.Reference,
+		StartsAt:      booking.StartsAt,
+		EndsAt:        booking.EndsAt,
+		Timezone:      tenant.Timezone,
+		Locale:        tenant.Locale,
+		ManagementURL: s.managementURL(booking),
+	})
+}