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_opportunity_writer.py

@@ -8,8 +8,8 @@ import pytest
 
 from arbitrade.config.settings import get_settings
 from arbitrade.detection.engine import OpportunityEvent
-from arbitrade.storage.pg_store import PgStore
 from arbitrade.storage.opportunities import AsyncOpportunityWriter
+from arbitrade.storage.pg_store import PgStore
 from arbitrade.storage.repositories import OpportunityRepository
 
 pytestmark = pytest.mark.integration