Add Last.fm scrobbling integration

Summary

• Scrobble tracks to Last.fm as you listen during the sorting workflow
• OAuth-style auth flow via Settings page (connect/disconnect)
• Automatic now-playing updates on track start, scrobble after 50% playback or 4 minutes

Changes

• New settings DB table for storing Last.fm session key
• Last.fm API client with signature generation (src/lib/server/api/lastfm.ts)
• 6 API routes under /api/lastfm/ (status, auth, callback, disconnect, now-playing, scrobble)
• Client-side scrobble tracker (src/lib/scrobble.ts) hooked into both list and random views
• Last.fm connection card on Settings page
• LASTFM_API_KEY and LASTFM_API_SECRET env vars

Setup

1. Add LASTFM_API_KEY and LASTFM_API_SECRET to env/docker-compose
2. Connect via Settings page

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