feat: autotag mixed-bag splitting, search relevance fixes, and multi-library download imports

Autotag: detect "junk drawer" folders with no artist/album consensus
and split them into synthetic per-cluster groups instead of forcing
one match on an unrelated pile of tracks; repair tagging_items rows
left behind by a prior scan orphan-cleanup gap.

Explore: fix an exact artist-name search being drowned out by its own
catalog entries in intent-prior scoring, and prune stale in_library
bookkeeping left behind when a referenced library row is deleted.

Download: fix a multi-library regression where every import failed
with "no library root configured" — the importer resolved the
library root from a legacy single-library config field that nothing
populates in the current multi-library model. It now resolves the
destination library per-request from the request's own library_id.
Also widen the Soulseek search window (12s -> 20s), measured against
real request history to be missing available peers on live queries.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2Agd9af5hE7qzti2ackiS

backend/database/sql/sqlcgen/artist_credit.sql.go

@@ -78,6 +78,38 @@ func (q *Queries) GetArtistCreditByText(ctx context.Context, text string) (Artis
 	return i, err
 }
 
+const getOrphanedArtistCreditIDs = `-- name: GetOrphanedArtistCreditIDs :many
+SELECT ac.id FROM artist_credit ac
+WHERE NOT EXISTS (SELECT 1 FROM recordings r WHERE r.artist_credit_id = ac.id)
+  AND NOT EXISTS (SELECT 1 FROM release_groups rg WHERE rg.album_artist_credit_id = ac.id)
+`
+
+// Artist credits no longer used by any recording or release group - run
+// after orphaned recordings/release groups are deleted, so a credit
+// that only existed for now-removed tracks is cleaned up too.
+func (q *Queries) GetOrphanedArtistCreditIDs(ctx context.Context) ([]int64, error) {
+	rows, err := q.db.QueryContext(ctx, getOrphanedArtistCreditIDs)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []int64
+	for rows.Next() {
+		var id int64
+		if err := rows.Scan(&id); err != nil {
+			return nil, err
+		}
+		items = append(items, id)
+	}
+	if err := rows.Close(); err != nil {
+		return nil, err
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const updateArtistCredit = `-- name: UpdateArtistCredit :exec
 UPDATE artist_credit 
 SET text = ?