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/app.go

@@ -182,6 +182,16 @@ func NewYellowJacketApp(
 	yjApp.library.SetJobRegistry(yjApp.jobs)
 	yjApp.explore.SetJobRegistry(yjApp.jobs)
 
+	// Let the release prefetch skip albums the user already owns in
+	// full — those open with no catalog call at all, so warming their
+	// tracklists spends the most expensive request in the app on
+	// nothing.  Injected because neither package imports the other.
+	yjApp.explore.SetAlbumComplete(func(albumID int64) bool {
+		c, err := yjApp.library.GetAlbumCompleteness(albumID)
+
+		return err == nil && c.Known && c.Complete
+	})
+
 	// create autotag service (depends on explore + tagWriter)
 	yjApp.autotag = autotagservice.NewService(
 		yjApp.logger.WithGroup("autotag"),
@@ -697,7 +707,13 @@ func (yj *YellowJacketApp) startJanitor() {
 		yj.database, coversDir, library.CoverArtFileSet,
 	))
 	yj.janitor.Register(maintenance.OrphanedArtistImagesJob(
-		yj.database, filepath.Join(dataDir, explore.ArtistImageDirName),
+		yj.database,
+		filepath.Join(dataDir, explore.ArtistImageDirName),
+		explore.ArtistImageDir,
+	))
+	yj.janitor.Register(maintenance.StrayArtistImageFilesJob(
+		filepath.Join(dataDir, explore.ArtistImageDirName),
+		explore.ArtistImageKeepNames(),
 	))
 	yj.janitor.Register(maintenance.ExpiredProxyCacheJob(
 		filepath.Join(dataDir, explore.CoverArtCacheDirName),