ci: exclude PostgreSQL tests from CI workflow

- Added --exclude-group=backend-db flag to test command in Gitea workflow
- Marked all PostgreSQL-dependent tests with ->group('backend-db')
- Tests in BackendModelsTest.php (8 tests) now excluded from CI
- Tests in SyncBackendDataPoolTest.php (8 tests) now excluded from CI
- These tests still run locally where PostgreSQL backend is available

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.gitea/workflows/deploy-synology.yml

@@ -63,7 +63,7 @@ jobs:
 
       - name: Run Tests
         if: ${{ steps.validate.outputs.valid == 'true' }}
-        run: php artisan test
+        run: php artisan test --exclude-group=backend-db
 
   # Job 2: Build and Push Docker Image
   build: