Opt-in error tracking that forwards uncaught errors to Sentry/Bugsnag-style services when ERROR_TRACKING_DSN env var is set. Without DSN, disabled. Features: - Sentry envelope format for wire compatibility - Express error middleware (drop-in after routes) - capture() + captureMessage() + flush() - Non-blocking — tracking errors never crash the app - 5s timeout on network sends - Includes hostname, node version, memory, uptime, request context 10 tests, 1633 total pass.