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_postgresql_schema.py

@@ -13,7 +13,7 @@ from contextlib import asynccontextmanager
 import pytest
 import pytest_asyncio
 
-from arbitrade.config.settings import Settings, get_settings
+from arbitrade.config.settings import get_settings
 from arbitrade.storage.pg_store import PgStore
 
 pytestmark = pytest.mark.integration
@@ -332,6 +332,7 @@ async def test_audit_list_recent(pg: PgStore) -> None:
     """AuditRepository.list_recent returns records in desc order."""
     await pg.migrate()
     from datetime import UTC, datetime
+
     from arbitrade.storage.repositories import AuditRecord, AuditRepository
 
     repo = AuditRepository(pg)