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_artists.sql.go

@@ -45,6 +45,16 @@ func (q *Queries) DeleteArtistCreditArtist(ctx context.Context, id int64) error
 	return err
 }
 
+const deleteArtistCreditArtistByCredit = `-- name: DeleteArtistCreditArtistByCredit :exec
+DELETE FROM artist_credit_artist
+WHERE credit_id = ?
+`
+
+func (q *Queries) DeleteArtistCreditArtistByCredit(ctx context.Context, creditID int64) error {
+	_, err := q.db.ExecContext(ctx, deleteArtistCreditArtistByCredit, creditID)
+	return err
+}
+
 const getArtistCreditArtist = `-- name: GetArtistCreditArtist :one
 SELECT id, artist_id, credit_id FROM artist_credit_artist 
 WHERE id = ? LIMIT 1