fix(art): take track ID from match info instead of item mb_trackid

[Samik081]

Summary

Fixes #38 — album art was not embedded and the import log showed:

beatport4: Failed to fetch track : Error 404 for '/v4/catalog/tracks//'
beatport4: No image data for track ; skipping

Root cause

The import_task_files art hook read the Beatport track ID from items[0].get("mb_trackid"). Other plugins can blank that field on the shared in-memory Item objects before the hook runs — notably zero configured with mb_trackid in its fields and update_database: true, whose write listener sets item[field] = None during the import write phase. The hook then requested /v4/catalog/tracks// (empty ID), got a 404, and skipped art embedding. Reproduced exactly with zero + scrub enabled on both fresh imports and -L re-imports.

Fix

Take the track ID from task.match.info — data the plugin itself produced — instead of the mutable library item:

• AlbumInfo matches: first track with a track_id (all tracks on a release share the same cover art)
• singleton TrackInfo matches: info.track_id
• no ID in the match: skip with a debug log instead of firing a bogus request

Testing

• 3 new regression tests (blanked mb_trackid, singleton path, match without track IDs); full suite passes 140/140
• Verified end-to-end against the live Beatport API in an isolated BEETSDIR with zero (mb_trackid zeroed, update_database: true) + scrub enabled: re-import of the reporter's release (4194963) fetches and embeds art in all 4 files; singleton import path verified as well