feat(tests): increase test coverage from 68% to 78%

Add comprehensive integration and API tests:
- Integration tests for SQLAlchemyPostRepository (34 tests)
- API tests for posts endpoints and error handlers (22 tests)
- Unit tests for PublishPostUseCase and ListPostsUseCase
- Unit tests for SessionTransactionManager

Also register generic exception handler in error_handler.py

All 167 tests pass, coverage now meets CI threshold of 70%

app/infrastructure/middleware/error_handler.py

@@ -129,3 +129,4 @@ def register_exception_handlers(app: FastAPI) -> None:
 
     app.add_exception_handler(DomainException, domain_exception_handler)  # type: ignore[arg-type]
     app.add_exception_handler(StarletteHTTPException, http_exception_handler)  # type: ignore[arg-type]
+    app.add_exception_handler(Exception, generic_exception_handler)