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/lyrics.go

@@ -133,6 +133,18 @@ func (e *Service) BackfillLibraryLyrics() {
 func (e *Service) backfillLibraryLyrics(ctx context.Context) {
 	total := 0
 
+	// LRCLIB has its own limiter, so this starves nothing — but it is
+	// still work nobody asked for, and it was the last background pass
+	// with no way to see or stop it.  The job is registered lazily,
+	// after the first batch proves there is something to do, because on
+	// a covered library every launch would otherwise put an empty job
+	// in the indicator.
+	ctx = WithBackgroundPriority(ctx)
+
+	var job *backfillJob
+
+	defer func() { job.finish(ctx) }()
+
 	for range lyricsBackfillMaxPasses {
 		if ctx.Err() != nil {
 			return
@@ -149,13 +161,24 @@ func (e *Service) backfillLibraryLyrics(ctx context.Context) {
 			break
 		}
 
+		if job == nil {
+			job, ctx = startBackfillJob(
+				ctx, e.index.jobRegistry(), lyricsBackfillJobID,
+				"Looking up lyrics",
+				"Tracks in your library with no lyrics yet",
+				len(candidates),
+			)
+		}
+
 		filled := 0
 
-		for _, c := range candidates {
+		for i, c := range candidates {
 			if ctx.Err() != nil {
 				return
 			}
 
+			job.progress(i, len(candidates))
+
 			if e.fetchAndStoreLyrics(ctx, c) != nil {
 				filled++
 				total++