feat(activity): per-tenant audit feed in notification dropdown

activity_events records booking lifecycle transitions with the actor
behind each change — staff (dashboard), customer (self-service), or
system (auto-promotion). The notification dropdown leads with the real
feed instead of derived recent-booking guesses.

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

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

apps/backend/internal/catalog/service.go

@@ -1365,6 +1365,16 @@ func (s *Service) CancelBooking(ctx context.Context, principal domain.Principal,
 		return err
 	}
 	_ = s.repo.CancelPendingReminderJobsForBooking(ctx, booking.ID)
+	_ = s.repo.AppendActivityEvent(ctx, db.ActivityEventParams{
+		TenantID:     membership.Tenant.ID,
+		ActorKind:    "staff",
+		ActorSubject: principal.Subject,
+		ActorName:    actorDisplayName(principal),
+		Action:       "booking.cancelled",
+		EntityType:   "booking",
+		EntityID:     &booking.ID,
+		Summary:      booking.CustomerName + " — " + s.bookingLabel(ctx, booking),
+	})
 
 	go s.sendBookingEmail(booking, false)
 
@@ -1395,10 +1405,46 @@ func (s *Service) PromoteBooking(ctx context.Context, principal domain.Principal
 		}
 		return err
 	}
+	_ = s.repo.AppendActivityEvent(ctx, db.ActivityEventParams{
+		TenantID:     membership.Tenant.ID,
+		ActorKind:    "staff",
+		ActorSubject: principal.Subject,
+		ActorName:    actorDisplayName(principal),
+		Action:       "booking.promoted",
+		EntityType:   "booking",
+		EntityID:     &promoted.ID,
+		Summary:      promoted.CustomerName + " — " + s.bookingLabel(ctx, promoted),
+	})
 	go s.sendBookingEmail(promoted, true)
 	return nil
 }
 
+// bookingLabel resolves the display label for a booking — service name for
+// appointments, session title for classes.
+func (s *Service) bookingLabel(ctx context.Context, booking db.BookingRecord) string {
+	if booking.ServiceID != nil {
+		if service, err := s.repo.GetServiceByID(ctx, *booking.ServiceID); err == nil {
+			return service.Name
+		}
+	}
+	if booking.ClassSessionID != nil {
+		if session, err := s.repo.GetClassSessionByID(ctx, *booking.ClassSessionID); err == nil {
+			return session.Title
+		}
+	}
+	return ""
+}
+
+func actorDisplayName(principal domain.Principal) string {
+	if principal.Name != "" {
+		return principal.Name
+	}
+	if principal.Email != "" {
+		return principal.Email
+	}
+	return principal.Subject
+}
+
 // sendBookingEmail builds the shared payload and fires a confirmation
 // (promoted=true) or cancellation notification. Failures are logged-and-done:
 // email delivery must not fail the cancellation itself.
@@ -1419,17 +1465,8 @@ func (s *Service) sendBookingEmail(booking db.BookingRecord, promoted bool) {
 		brandColor = "#a65c3e"
 	}
 
-	var serviceName, locationName string
-	if booking.ServiceID != nil {
-		if service, err := s.repo.GetServiceByID(ctx, *booking.ServiceID); err == nil {
-			serviceName = service.Name
-		}
-	}
-	if booking.ClassSessionID != nil {
-		if session, err := s.repo.GetClassSessionByID(ctx, *booking.ClassSessionID); err == nil {
-			serviceName = session.Title
-		}
-	}
+	var locationName string
+	serviceName := s.bookingLabel(ctx, booking)
 	if booking.LocationID != nil {
 		if location, err := s.repo.GetLocationByID(ctx, *booking.LocationID); err == nil {
 			locationName = location.Name