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/artist_credit.sql

@@ -32,3 +32,11 @@ SELECT
   (SELECT COUNT(*) FROM recordings WHERE artist_credit_id = ?1) +
   (SELECT COUNT(*) FROM release_groups WHERE album_artist_credit_id = ?1)
 AS total;
+
+-- name: GetOrphanedArtistCreditIDs :many
+-- 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.
+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);