test: add coverage tests across backend + fix mock for PENDING checkout support

New test files cover previously untested paths across DAV, validators,
S3, Square, mw, bookings, user, and payments packages.

Includes mock fix: HoldCheckouts flag on MockClient allows tests to
pause auto-complete goroutine for testing PENDING checkout states.

Coverage: 50.4% → 65.0% (+14.6pp)

backend/handlers/notifications/notifications_test.go

@@ -698,3 +698,39 @@ func TestAcknowledgePendingBookingNotification_NonTxCaller(t *testing.T) {
 		t.Errorf("expected no error for non-tx caller, got: %v", err)
 	}
 }
+
+func TestAcknowledgePendingBookingNotification_ExecError(t *testing.T) {
+	t.Parallel()
+	ctx, tx := testutils.SetupTestTx(t)
+
+	userID, err := fixtures.CreateTestUser(tx)
+	if err != nil {
+		t.Fatalf("failed to create user: %v", err)
+	}
+
+	serviceID, err := fixtures.CreateTestService(tx)
+	if err != nil {
+		t.Fatalf("failed to create service: %v", err)
+	}
+
+	bookingID, err := fixtures.CreateTestBookingAtTime(tx, userID, serviceID, clock.Now().Add(24*time.Hour))
+	if err != nil {
+		t.Fatalf("failed to create booking: %v", err)
+	}
+
+	_, err = tx.Exec(ctx, `
+		INSERT INTO admin_notifications (booking_id, reason, acknowledged_at)
+		VALUES ($1, 'pending_booking', NULL)
+	`, bookingID)
+	if err != nil {
+		t.Fatalf("failed to create notification: %v", err)
+	}
+
+	cancelCtx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	err = AcknowledgePendingBookingNotification(tx, cancelCtx, bookingID)
+	if err == nil {
+		t.Error("expected error from cancelled context, got nil")
+	}
+}