feat: update environment configuration and improve repository handling

- Added PG_PASSWORD to .env.example for database connection.
- Removed unnecessary imports and streamlined code in various modules.
- Enhanced error handling in ConfigSettingRepository and ConfigPairingRepository.
- Updated test files to remove unused imports and improve clarity.

tests/integration/test_audit_repository.py

@@ -5,7 +5,6 @@ from contextlib import asynccontextmanager
 from datetime import UTC, datetime
 
 import pytest
-import pytest_asyncio
 
 from arbitrade.config.settings import get_settings
 from arbitrade.storage.pg_store import PgStore