fix(tests): protect dev admin from test cleanup, use isolated test names

Tests were deleting the development admin user, causing ON DELETE SET NULL
to cascade and set created_by_admin_id to NULL on all libraries.

- test_helpers: skip deletion of testuser@tests.bookhoard.internal
- sync_integration_test: use test-sync% prefix for isolated test data

cmd/server/tests/test_helpers_test.go

@@ -558,11 +558,15 @@ func setupTestServer(t *testing.T) *TestServerSetup {
 
 	ctx := context.Background()
 
-	// Delete ALL test users (any user with test email domains) to ensure clean state
-	// This handles users created during tests that may have been promoted to admin, etc.
+	// Delete transient test users but preserve the dev admin user
+	// testuser@tests.bookhoard.internal is the shared dev admin — deleting it
+	// triggers ON DELETE SET NULL on libraries.created_by_admin_id
 	allUsers, err := queries.ListUsers(ctx)
 	if err == nil {
 		for _, user := range allUsers {
+			if user.Email == "testuser@tests.bookhoard.internal" {
+				continue
+			}
 			if strings.HasSuffix(user.Email, "@example.com") || strings.HasSuffix(user.Email, "@tests.bookhoard.internal") {
 				queries.DeleteUser(ctx, user.ID)
 			}