Four enrichment workers (Last.fm, MusicBrainz, Tidal, Qobuz) had a
bug where every background loop re-processed the same rows because
the existing-ID short-circuit path never set match_status, and two
workers queried the wrong column when checking for an existing ID.
lastfm_worker._get_existing_id queried a non-existent lastfm_id
column; the real column is lastfm_url. The method now reads
lastfm_url for all three entity types.
musicbrainz_worker._get_existing_id queried musicbrainz_id for all
entity types, but albums use musicbrainz_release_id and tracks use
musicbrainz_recording_id. The method now uses a per-type column map.
All four workers (lastfm, musicbrainz, tidal, qobuz) now write
match_status='matched' when they short-circuit on an already-present
external ID, so these rows are no longer re-selected on the next
worker sweep.
A new migration (_backfill_match_status_for_existing_ids) runs once
on startup to retroactively set match_status='matched' for rows that
already have an external ID but NULL match_status. This covers legacy
data, manual matches, and rows populated from file tags outside the
worker.
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