perf(explore): make the owned-artist backfill yield, mark, and stop

The post-scan backfills share MusicBrainz's rate limiters with every
page the user can open, and both were FIFO — so a thousand-artist
enrichment put an album page behind an hour of queued work.
WithBackgroundLane/WithBackgroundPriority add a slower second lane: a
marked wait takes no token while any interactive wait is outstanding.
It is a context marker rather than a parameter because a backfill calls
the same client methods a detail page does. A long backfill also has to
be visible and stoppable, so jobs.KindCatalogEnrich registers both with
progress and cancel — after the work is counted, since these passes are
a no-op on every launch once the library is covered.

What it does not fetch is the point. It ran for hours against a
900-artist library and marked nothing, because three of the four things
it did per artist were work nobody asked for: similar artists, which
the artist page already resolves on view, and a full GetArtistImage
(fanart.tv, TheAudioDB, Wikidata, Wikipedia, ten portraits) reached
only to warm the MB artist lookup EnsureArtistRels does alone. It was
also serial across artists while every limiter is per-host and idle.

The marks are a table rather than more explore_index columns, because
artifactimport merges by column list and a flag added there is a second
place to remember. BrowseReleaseGroupsAll pages to exhaustion, where
the old call silently cut a prolific artist at 100 release groups.

One portrait is downloaded now; the rest are remembered as URLs.
resolveAllSources downloaded every candidate, up to ten, full size,
while nothing reads anything but primary.jpg — 5.3 GB measured on a
real cache, 4.1 GB of it unreachable. OrphanedArtistImagesJob is why
that survived: it joined the bare MBID onto the images directory, but
artist directories are sharded under a two-character prefix, so it
named a path that never existed and deleted the rows that were the only
record of the files it left behind.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UDCbcCZQepnpSQYJ6SxxZm

backend/database/sql/sqlcgen/models.go

@@ -26,6 +26,12 @@ type ArtistCreditArtist struct {
 	CreditID int64
 }
 
+type ArtistEnrichment struct {
+	ArtistMbid string
+	BrowsedAt  sql.NullTime
+	SimilarAt  sql.NullTime
+}
+
 type ArtistImage struct {
 	ID         int64
 	ArtistMbid string