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/crud.go

@@ -116,6 +116,16 @@ func (l *Library) AddLibrary(path string) (*sqlcgen.Library, error) {
 	return &lib, nil
 }
 
+// LibraryPath resolves a library's root directory by id.
+func (l *Library) LibraryPath(id int64) (string, error) {
+	lib, err := l.db.ReadQueries.GetLibrary(l.ctx, id)
+	if err != nil {
+		return "", fmt.Errorf("could not get library %d: %w", id, err)
+	}
+
+	return lib.Path, nil
+}
+
 // RenameLibrary validates and updates a library's display name.
 func (l *Library) RenameLibrary(id int64, newName string) error {
 	newName = strings.TrimSpace(newName)