Replace raw database/sql with jmoiron/sqlx for cleaner query handling.
Support both SQLite (default) and PostgreSQL as configurable backends.
Configuration via:
- CLI flags: -database-driver, -database-path, -database-url
- Environment: PROXY_DATABASE_DRIVER, PROXY_DATABASE_PATH, PROXY_DATABASE_URL
- Config file: database.driver, database.path, database.url
Tests run against both databases when PROXY_DATABASE_URL is set.