Add Sentry error tracking and performance tracing

[jakebromberg]

Context

After reverting Sentry from dj-site (Cloudflare Workers incompatibility), neither repo has error tracking. Backend-Service runs on EC2 with no compatibility issues, so we use @sentry/node for server-side error capture and performance tracing.

Changes

Sentry Initialization

• apps/backend/instrument.ts and apps/auth/instrument.ts -- Sentry.init() with DSN, release, environment, and tracesSampleRate: 1.0. Auto-detected integrations cover Express, HTTP, and PostgreSQL.

Express Integration

• apps/backend/app.ts -- import './instrument.js' as first line, Sentry.setupExpressErrorHandler(app) after routes before errorHandler, requestIdMiddleware after express.json().
• apps/auth/app.ts -- import './instrument.js' as first line, Sentry.setupExpressErrorHandler(app) after routes, new fallback error handler (auth had none).

Cross-Service Correlation

• apps/backend/middleware/requestId.ts -- Reads X-Request-Id from incoming request (sent by dj-site) or generates one via crypto.randomUUID(). Sets on response header and tags Sentry scope with request_id.
• CORS updated -- X-Request-Id added to allowedHeaders and exposedHeaders.

Build Configuration

• apps/backend/tsup.config.ts -- external: ['@sentry/node'], sourcemap: true (Sentry monkey-patches require unbundled modules).
• apps/auth/tsup.config.ts -- '@sentry/node' added to existing external array.

Deploy

• .github/actions/deploy-service/action.yml -- -e SENTRY_RELEASE=$TARGET_APP@$DEPLOY_TAG added to docker run.

Tests

• tests/unit/middleware/requestId.test.ts -- 5 tests: UUID generation, pass-through of existing header, response header setting, Sentry scope tagging.

Environment Variables

Add to .env on EC2:

• SENTRY_DSN -- Sentry project DSN

Verification

1. npm run build succeeds for both workspaces
2. npm run test:unit passes (125 tests, including 5 new requestId tests)
3. Deploy to EC2, hit /healthcheck -- confirm X-Request-Id response header
4. Trigger an error -- confirm it appears in Sentry with request_id tag
5. Check Sentry Performance tab for Express spans