release: finalize portfolio MVP v1.2.1

[s194586]

What changed

• Added portfolio UI screenshots.
• Improved the recruiter-facing README and removed duplicated release presentation.
• Added the Beyond MVP backlog.
• Aligned release-facing versions to v1.2.1.
• Refreshed uv.lock to podcast-shorts-cutter v1.2.1.
• Enabled CI on pull requests to main.
• Aligned the frontend Docker image version.

Validation

• uv lock --check — passed.
• uv sync --locked — passed.
• uv run python -m unittest discover -s tests — 604 tests passed, 3 skipped.
• uv pip check — passed; all installed packages are compatible.
• npm ci — passed.
• npm run test -- --run — 55 tests passed.
• npm run lint — passed.
• npm run build — passed.
• git diff --check — passed.

Security note

npm audit --omit=dev still reports 2 high-severity react-router advisories concerning React Router RSC Mode CSRF. The MVP uses BrowserRouter and Routes and does not use React Router RSC APIs, so the affected RSC path is not used by the current application. Full npm audit reports 4 vulnerabilities total: those 2 runtime advisories plus 1 moderate postcss and 1 high undici advisory in the development dependency tree. npm audit fix --force was not run.

Release note

After merge: GitHub Release v1.2.1 — Portfolio MVP.