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/explore/assetdirs.go

@@ -1,5 +1,7 @@
 package explore
 
+import "path/filepath"
+
 // Asset directory names under the user data directory.
 //
 // These are exported so the maintenance janitor can sweep them without
@@ -18,3 +20,34 @@ const (
 	// the database references these files.
 	CoverArtCacheDirName = "cover-art-cache"
 )
+
+// ArtistImageDir returns the directory holding one artist's images.
+//
+// Artist directories are sharded under the MBID's first two characters,
+// which the janitor cannot be expected to know and had got wrong — so
+// this is the one definition of that layout, and ArtistImageProvider's
+// own artistDir defers to it.
+func ArtistImageDir(baseDir, mbid string) string {
+	if len(mbid) < 2 {
+		return filepath.Join(baseDir, "xx", mbid)
+	}
+
+	return filepath.Join(baseDir, mbid[:2], mbid)
+}
+
+// ArtistImageKeepNames is every file an artist's directory is meant to
+// hold: the portrait, its three size tiers, and the marker recording
+// that no portrait was found.  Anything else is a downloaded candidate
+// from a version that kept all of them (see StrayArtistImageFilesJob).
+func ArtistImageKeepNames() map[string]bool {
+	keep := map[string]bool{
+		"primary.jpg":  true,
+		artistMissFile: true,
+	}
+
+	for _, tier := range artistImageTiers {
+		keep["primary"+tier.Suffix+".jpg"] = true
+	}
+
+	return keep
+}