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.

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

apps/backend/internal/bookings/lifecycle.go

@@ -55,6 +55,14 @@ func PromoteWaitlist(ctx context.Context, repo db.Repository, tenantID, sessionI
 		}
 		confirmed++
 		promoted = append(promoted, booking)
+		_ = repo.AppendActivityEvent(ctx, db.ActivityEventParams{
+			TenantID:   tenantID,
+			ActorKind:  "system",
+			Action:     "booking.promoted",
+			EntityType: "booking",
+			EntityID:   &booking.ID,
+			Summary:    booking.CustomerName + " — " + session.Title,
+		})
 	}
 	return promoted, nil
 }