Consolidate CI/CD: tests, typecheck, preview + prod deploys

Goal
- Single PR workflow: lint, typecheck, tests, preview deploy.
- Single push-to-main workflow: production deploy.

Tasks
- Add typecheck job.
- Cache installs/builds.
- Ensure only one deploy path is active (Actions or Vercel Git).

Acceptance Criteria
- Required checks green before merge.
- No duplicate deploys on PR or main.