Findings now carry artist_thumb_url alongside album_thumb_url (same key the
track-repair findings use, so the findings UI already renders it).
Fetched via a guarded _lookup_artist_thumb() — checks the artists table has a
thumb_url column first and swallows any error — rather than adding ar.thumb_url
to the shared load_album_and_tracks SELECT. The shared-loader approach was
tried first and REVERTED: it crashed reorganize on schemas whose artists table
has no thumb_url column (caught by 40 orchestrator tests). The lookup only runs
for albums that actually resolve, so it adds no cost to the no-source-id
short-circuit majority.
Tests: orchestration test asserts artist_name + album_thumb_url + artist_thumb_url
flow through. 47 canonical + 104 canonical/reorganize regression tests pass.
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