feat(bookings): customer self-service rebooking with real slot picker

CustomerBookingView now carries tenantSlug/serviceId/staffId/
classSessionId so the manage page can query live availability; class
bookings resolve their session title as the display label. Reschedule
rejects waitlisted/completed bookings and session-bound class bookings
(400), collapsing a dead branch in the appointment path.

The manage page reschedule dialog replaces free-form date/time inputs
with a date → availability-slot picker filtered to appointment mode,
so customers can only move to a genuinely free slot with the correct
duration; 409 surfaces "slot just taken". Actions are status-gated:
reschedule for confirmed/pending appointments, cancel for live
bookings, explanatory note for class bookings.

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

@@ -73,6 +73,12 @@ func (s *CustomerService) managementURL(booking db.BookingRecord) string {
 }
 
 func (s *CustomerService) descriptors(ctx context.Context, booking db.BookingRecord) (serviceName string, locationName string) {
+	if booking.ServiceID == nil && booking.ClassSessionID != nil {
+		// Class bookings carry no service; the session title is the label.
+		if session, err := s.repo.GetClassSessionByID(ctx, *booking.ClassSessionID); err == nil {
+			serviceName = session.Title
+		}
+	}
 	if booking.ServiceID != nil {
 		if services, err := s.repo.ListServicesByTenant(ctx, booking.TenantID); err == nil {
 			for _, service := range services {
@@ -119,6 +125,10 @@ func (s *CustomerService) GetBookingByReference(ctx context.Context, reference s
 		CustomerEmail:    booking.CustomerEmail,
 		Service:          serviceName,
 		BusinessName:     tenant.Name,
+		TenantSlug:       tenant.Slug,
+		ServiceID:        booking.ServiceID,
+		StaffID:          booking.StaffID,
+		ClassSessionID:   booking.ClassSessionID,
 		StartsAt:         booking.StartsAt,
 		EndsAt:           booking.EndsAt,
 		Location:         locationName,
@@ -142,6 +152,18 @@ func (s *CustomerService) RescheduleBooking(ctx context.Context, reference strin
 		return ErrBookingCancelled
 	}
 
+	// Only live bookings may move: waitlisted entries sit in a queue tied to
+	// their session, and completed bookings are history.
+	if booking.Status == "waitlisted" || booking.Status == "completed" {
+		return ErrInvalidReschedule
+	}
+
+	// Class bookings are bound to their session window; moving to another
+	// session means cancelling and booking again.
+	if booking.ClassSessionID != nil {
+		return ErrInvalidReschedule
+	}
+
 	newStartsAt, err := time.Parse(time.RFC3339, req.NewStartsAt)
 	if err != nil {
 		return ErrInvalidReschedule
@@ -174,16 +196,14 @@ func (s *CustomerService) RescheduleBooking(ctx context.Context, reference strin
 		}
 	}
 
-	// Appointments must land inside bookable hours and off blocked days;
-	// class bookings keep their session-defined window.
-	if booking.ClassSessionID == nil {
-		tenant, err := s.repo.GetTenantByID(ctx, booking.TenantID)
-		if err != nil {
-			return err
-		}
-		if err := validateAppointmentTarget(ctx, s.repo, tenant, booking.StaffID, booking.LocationID, newStartsAt, newEndsAt); err != nil {
-			return ErrInvalidReschedule
-		}
+	// Appointments must land inside bookable hours and off blocked days.
+	// Class bookings never reach this point — rejected above.
+	tenant, err := s.repo.GetTenantByID(ctx, booking.TenantID)
+	if err != nil {
+		return err
+	}
+	if err := validateAppointmentTarget(ctx, s.repo, tenant, booking.StaffID, booking.LocationID, newStartsAt, newEndsAt); err != nil {
+		return ErrInvalidReschedule
 	}
 
 	if err := s.repo.RescheduleBooking(ctx, booking.ID, newStartsAt, newEndsAt); err != nil {