feat: add Jellyfin playlist support alongside Plex

[ianchesal]

Summary

The app currently manages a "Date Night" playlist exclusively via Plex. Adding Jellyfin support would let users running Jellyfin instead of (or alongside) Plex get the same automatic playlist sync.

Motivation

Jellyfin is the dominant open-source, self-hosted alternative to Plex. Many *arr stack setups use Jellyfin. Right now those users get no playlist management out of the box.

Proposed approach

• Add JELLYFIN_URL and JELLYFIN_API_KEY env vars (Jellyfin uses API key auth, not a token)
• Create src/lib/jellyfin.ts mirroring the interface of src/lib/plex.ts — create/update a "Date Night" playlist, sync available movies in watch order
• Update src/lib/sync.ts to call whichever clients are configured (both can run simultaneously if both sets of env vars are present)
• Update .env.example and the README configuration table
• Graceful degradation: if JELLYFIN_URL / JELLYFIN_API_KEY are not set, Jellyfin sync is skipped silently — same pattern as Plex today

Open questions

• Jellyfin playlist API differs from Plex — needs investigation into how playlists are created and items matched by media ID
• Should matching be done by IMDB ID (most reliable) or title? Jellyfin exposes provider IDs per item.