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.go

@@ -790,6 +790,14 @@ func (m *Manager) grab(
 		opts := m.importOptions()
 		opts.WriteTags = true
 
+		opts.LibraryRoot, err = m.library.LibraryPath(req.LibraryID)
+		if err != nil {
+			m.failItem(ctx, job, item, req.ID,
+				fmt.Errorf("resolve library root: %w", err))
+
+			return
+		}
+
 		imported, err = m.importer.Import(ctx, req, result, opts)
 		if err != nil {
 			m.failItem(ctx, job, item, req.ID, err)