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/library/leak_test.go

@@ -16,6 +16,8 @@ var staleTolerated = map[string]string{
 		"stale entries are filtered by joining track_metadata and are " +
 		"cleared by a full rescan",
 	"lyrics_index": "contentless FTS5, same constraint as search_index",
+	"schema_migrations": "global migration bookkeeping, not scoped to any " +
+		"library; removing the only library must not touch it",
 }
 
 // Removing the only library must leave no owned or derived rows behind.