feat: add PostgreSQL CI integration tests (#156)

* feat: add PostgreSQL CI integration tests

Add --storage-backend pytest option and shared storage_backend fixture
in conftest.py that creates SQLiteBackend or PostgreSQLBackend based
on the flag. Migrate 13 storage test files to use shared fixture
instead of local SQLiteBackend fixtures.

Add test-postgres CI job with PostgreSQL 17 service container that
runs the full test suite against real PostgreSQL.

* fix: use TRUNCATE CASCADE for PG cleanup, wrap in try/finally

TRUNCATE is faster than per-table DELETE and resets autoincrement
sequences. try/except ensures reset_storage() always runs even if
cleanup fails due to a corrupted connection from a failing test.

* fix: document _engine coupling in PG cleanup comment

tests/test_user_storage.py

@@ -2,16 +2,6 @@
 
 from __future__ import annotations
 
-import pytest
-
-from turnstone.core.storage._sqlite import SQLiteBackend
-
-
-@pytest.fixture()
-def db(tmp_path):
-    """Create a fresh SQLite backend for each test."""
-    return SQLiteBackend(str(tmp_path / "test.db"))
-
 
 class TestUserCRUD:
     def test_create_and_get(self, db):