Tech Debt: Implement CloudWatch alarms and operational monitoring

[pofallon]

Context

The README documents operational TODOs that were deferred during site-shell implementation (spec 002):

## Monitoring & Operational TODOs

- [ ] Connect the Amplify app to CloudWatch alarms that page on sustained 5xx errors from the SSR Lambda.
- [ ] Schedule the Playwright navigation suite via GitHub Actions (daily) and surface failures in Slack.
- [ ] Add Route 53 health checks for \`/\` and \`/blog\` once the Amplify branch is publicly reachable.
- [ ] Record error budget dashboards (Core Web Vitals + uptime) once traffic is available.

Tasks

CloudWatch Alarms

• Create CloudWatch alarm for SSR Lambda 5xx error rate
• Configure SNS topic for paging on sustained errors
• Set appropriate thresholds (e.g., >1% 5xx for 5 minutes)

Automated E2E Testing

• Create GitHub Actions workflow for daily Playwright runs
• Configure Slack webhook for failure notifications
• Run tests against staging/production URLs

Health Checks

• Create Route 53 health checks for / endpoint
• Create Route 53 health checks for /blog endpoint
• Configure alerting for health check failures

Observability Dashboard

• Set up Core Web Vitals tracking (LCP, FID, CLS)
• Create uptime monitoring dashboard
• Define error budget SLOs

Related Specs

• 002-nextjs-app-shell (README.md line 148-153)