feat: add per-show episode scan

[zeno-codebot]

Summary

• add a lightweight Sonarr series endpoint for episode-duration selectors
• allow the episode-duration check API to scope results to one Sonarr show
• add a show picker to the Episode Duration view and pass the selected series into the scan

Verification

• node --check backend/src/routes/sonarr.js
• node --check backend/src/routes/episodeDuration.js
• npm run build