feat(devops): Story 8.3 browser smoke and CI quality gate

[bychrisr]

Summary

• add Playwright smoke infrastructure for browser runtime journeys
• add learner smoke test (catalog -> playback) and admin smoke test (login + create event)
• add CI quality gate workflow for frontend builds + browser smoke on PR/push to main
• add explicit smoke gate script with actionable diagnostics paths on failures
• expand release:readiness gate to enforce browser smoke, frontend builds, and API regression checks
• update Story 8.3 to Review with QA evidence

Validation

• npm run e2e:smoke
• npm run e2e:smoke:gate
• npm run build --workspace @flix/web
• npm run build --workspace @flix/admin
• npm run release:readiness