Switch from mattn/go-sqlite3 (CGO) to modernc.org/sqlite (pure Go) for auth, audit, and notification queue storage. This enables SQLite functionality on arm64 Docker images which are built with CGO_ENABLED=0. Related to #1140
- Add SQLite-backed audit logger for persistent audit trails - Implement cryptographic signing for tamper detection - Add audit log export functionality - Add webhook notifications for audit events