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/release_groups.sql

@@ -167,6 +167,14 @@ ORDER BY rg.name;
 -- name: CountReleaseGroupRecordings :one
 SELECT COUNT(*) FROM release_group_recordings WHERE release_group_id = ?;
 
+-- name: GetOrphanedReleaseGroupIDs :many
+-- Release groups with no recordings left in them - run after orphaned
+-- recordings (and their release_group_recordings rows) are deleted, so
+-- a release group whose last owned track was removed is cleaned up too.
+SELECT rg.id FROM release_groups rg
+LEFT JOIN release_group_recordings rgr ON rgr.release_group_id = rg.id
+WHERE rgr.id IS NULL;
+
 -- name: GetAlbumsByArtistByLibrary :many
 SELECT
     rg.id,