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/explorefts_test.go

@@ -1,6 +1,7 @@
 package database
 
 import (
+	"strings"
 	"testing"
 )
 
@@ -157,3 +158,160 @@ func TestExploreFTSSuspendIsIdempotent(t *testing.T) {
 		t.Fatalf("resume: %v", err)
 	}
 }
+
+// ftsSegmentCount reports how much the FTS index itself has been
+// written to.  Every delete + insert the update trigger performs
+// appends to the shadow content table, so this is the observable that
+// tells "the trigger re-indexed the row" from "the trigger declined
+// to".  Search results cannot: a no-op re-index leaves the same
+// matches behind.
+func ftsSegmentCount(t *testing.T, db *DB) int {
+	t.Helper()
+
+	rows, err := db.QueryContext("SELECT COUNT(*) FROM explore_index_fts_data")
+	if err != nil {
+		t.Fatalf("fts data count: %v", err)
+	}
+
+	defer func() { _ = rows.Close() }()
+
+	n := 0
+
+	if rows.Next() {
+		if err := rows.Scan(&n); err != nil {
+			t.Fatalf("scan fts data count: %v", err)
+		}
+	}
+
+	return n
+}
+
+// The common write in this schema is an upsert whose merge rules keep
+// every existing value — the discography backfill re-browsing a known
+// artist, the incremental dump refreshing popularity.  Re-indexing
+// those cost an FTS5 delete against a multi-million row index while
+// holding the single writer connection, which is what starved the
+// playback path.  An update that leaves title, artist_name and aliases
+// alone must not touch the FTS index at all.
+func TestExploreFTSUpdateSkipsUnchangedText(t *testing.T) {
+	db := NewTestDB(t)
+
+	seedExploreRow(t, db, "mbid-1", "Unchanged Title", "Steady Artist")
+
+	before := ftsSegmentCount(t, db)
+
+	// A popularity refresh: an FTS column is not named at all.
+	if _, err := db.ExecContext(
+		"UPDATE explore_index SET popularity = 42 WHERE mbid = 'mbid-1'",
+	); err != nil {
+		t.Fatalf("popularity update: %v", err)
+	}
+
+	// An upsert-shaped write that re-states the text identically, which
+	// is what the merge rules produce for a row that has not changed.
+	if _, err := db.ExecContext(`
+		UPDATE explore_index
+		SET title = 'Unchanged Title', artist_name = 'Steady Artist', popularity = 43
+		WHERE mbid = 'mbid-1'
+	`); err != nil {
+		t.Fatalf("no-op text update: %v", err)
+	}
+
+	if got := ftsSegmentCount(t, db); got != before {
+		t.Errorf(
+			"FTS index written by an update that changed no text: %d rows, want %d",
+			got, before,
+		)
+	}
+
+	if got := ftsMatches(t, db, "Unchanged"); got != 1 {
+		t.Errorf("matches after unchanged updates = %d, want 1", got)
+	}
+}
+
+// The other half of the same guard: a real rename still re-indexes,
+// old term gone and new term found.
+func TestExploreFTSUpdateReindexesChangedText(t *testing.T) {
+	db := NewTestDB(t)
+
+	seedExploreRow(t, db, "mbid-2", "Original Title", "Some Artist")
+
+	if _, err := db.ExecContext(
+		"UPDATE explore_index SET title = 'Corrected Title' WHERE mbid = 'mbid-2'",
+	); err != nil {
+		t.Fatalf("rename: %v", err)
+	}
+
+	if got := ftsMatches(t, db, "Original"); got != 0 {
+		t.Errorf("matches for the old title = %d, want 0", got)
+	}
+
+	if got := ftsMatches(t, db, "Corrected"); got != 1 {
+		t.Errorf("matches for the new title = %d, want 1", got)
+	}
+
+	// The same for the other two indexed columns.
+	if _, err := db.ExecContext(
+		"UPDATE explore_index SET artist_name = 'Renamed Artist', aliases = 'AKA Thing' WHERE mbid = 'mbid-2'",
+	); err != nil {
+		t.Fatalf("artist rename: %v", err)
+	}
+
+	if got := ftsMatches(t, db, "Renamed"); got != 1 {
+		t.Errorf("matches for the new artist = %d, want 1", got)
+	}
+
+	if got := ftsMatches(t, db, "AKA"); got != 1 {
+		t.Errorf("matches for the new alias = %d, want 1", got)
+	}
+}
+
+// An existing install already carries the previous, unguarded trigger,
+// and a create that tolerated "already exists" would leave it there
+// forever — so the definition has to be replaced on open, not merely
+// offered.
+func TestExploreFTSTriggersAreReplacedOnOpen(t *testing.T) {
+	db := NewTestDB(t)
+
+	if err := db.SuspendExploreIndexFTS(); err != nil {
+		t.Fatalf("suspend: %v", err)
+	}
+
+	// The shape that shipped before: fires on every UPDATE.
+	if _, err := db.ExecContext(`
+		CREATE TRIGGER explore_index_au AFTER UPDATE ON explore_index BEGIN
+			INSERT INTO explore_index_fts(explore_index_fts, rowid, title, artist_name, aliases)
+			VALUES ('delete', old.id, old.title, old.artist_name, old.aliases);
+			INSERT INTO explore_index_fts(rowid, title, artist_name, aliases)
+			VALUES (new.id, new.title, new.artist_name, new.aliases);
+		END
+	`); err != nil {
+		t.Fatalf("install old trigger: %v", err)
+	}
+
+	if err := createExploreIndexFTSTriggers(db.Ctx, db.db); err != nil {
+		t.Fatalf("recreate triggers: %v", err)
+	}
+
+	rows, err := db.QueryContext(
+		"SELECT sql FROM sqlite_master WHERE type = 'trigger' AND name = 'explore_index_au'",
+	)
+	if err != nil {
+		t.Fatalf("read trigger sql: %v", err)
+	}
+
+	defer func() { _ = rows.Close() }()
+
+	definition := ""
+
+	if rows.Next() {
+		if err := rows.Scan(&definition); err != nil {
+			t.Fatalf("scan trigger sql: %v", err)
+		}
+	}
+
+	if !strings.Contains(definition, "UPDATE OF") ||
+		!strings.Contains(definition, "WHEN") {
+		t.Errorf("explore_index_au was not replaced; definition is:\n%s", definition)
+	}
+}