feat: extend deep scan to movie duration

[zeno-codebot]

Summary

• extend the PR feat: add Plex deep scan mode #12 deep-scan workflow to the movie duration check endpoint and response summary
• add the movie duration deep-scan toggle UI, persisted default hydration, and force-refresh compatibility
• document deep scan support for movie duration checks

Verification

• npm test in backend/
• node --check src/routes/movies.js src/api/plex.js in backend/
• npm run build in frontend/

[zeno-codebot]

@copilot review