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/queries/artists.sql

@@ -39,6 +39,15 @@ JOIN artist_credit ac ON ac.id = aca.credit_id
 JOIN release_groups rg ON rg.album_artist_credit_id = ac.id
 ORDER BY a.name;
 
+-- name: GetOrphanedArtistIDs :many
+-- Artists no longer credited on any recording or release group - left
+-- behind when a scan's orphan cleanup removes the audio_files that used
+-- to justify them, since deleting an audio_files row doesn't cascade.
+SELECT a.id FROM artists a
+WHERE NOT EXISTS (
+    SELECT 1 FROM artist_credit_artist aca WHERE aca.artist_id = a.id
+);
+
 -- name: GetAlbumArtistsByLibrary :many
 SELECT DISTINCT a.id, a.name, a.mbid
 FROM artists a