Add pydantic-settings based Settings class for loading configuration
from environment variables or .env file. Provides early-stage config
before database-backed config store is available.
Includes:
- CENTRAL_DB_DSN, CENTRAL_NATS_URL, CENTRAL_MASTER_KEY_PATH, CENTRAL_LOG_LEVEL
- Cached loader with get_settings()
- Tests for env vars, .env file, validation, caching
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>