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/download/manager_test.go

@@ -31,15 +31,15 @@ func newManagerFixture(t *testing.T) managerFixture {
 	store := NewStore(db)
 	staging := newTestStaging(t)
 	tags := newRecordingTagWriter()
-	lib := &stubLibrary{}
 	root := t.TempDir()
+	lib := &stubLibrary{path: root}
 
 	imp := NewImporter(slogDiscard(), staging, tags, lib)
 
 	m := NewManager(
 		slogDiscard(), store, NewMemSecretStore(), staging, imp, lib,
 	)
-	m.SetImportOptions(ImportOptions{LibraryRoot: root})
+	m.SetImportOptions(ImportOptions{})
 
 	return managerFixture{
 		manager: m,