Add Postgres table adapter with TableRepositoryInterface.

Introduce PostgresAdapter for dict-based row CRUD via psycopg3, including config, lazy exports, unit/integration tests with testcontainers, and an example UserTableRepository.

Co-authored-by: Cursor <[email protected]>

python_repositories/examples/user_table_repository.py

@@ -0,0 +1,19 @@
+"""Example domain repository backed by Postgres tables."""
+
+from typing import Any
+
+from python_repositories.adapters.postgres_adapter import PostgresAdapter
+
+
+class UserTableRepository(PostgresAdapter):
+    """Example: domain repository backed by a Postgres table."""
+
+    def get_user(self, user_id: str) -> dict[str, Any] | None:
+        return self.fetch_one(user_id)
+
+    def save_user(self, user_id: str, user: dict[str, Any]) -> None:
+        row = {self._config.primary_key: user_id, **user}
+        self.upsert(row)
+
+    def delete_user(self, user_id: str) -> None:
+        self.delete(user_id)