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/autotag/local.go

@@ -39,14 +39,16 @@ func (r *LocalResolver) LocalTracksForGroup(
 	out := make([]LocalTrack, 0, len(rows))
 	for _, row := range rows {
 		out = append(out, LocalTrack{
-			AudioFileID:   row.ID,
-			FilePath:      row.FilePath,
-			Title:         row.Title,
-			Artist:        row.ArtistName,
-			TrackNumber:   int(row.TrackNumber),
-			DiscNumber:    int(row.DiscNumber),
-			LengthMillis:  row.LengthMilliseconds,
-			RecordingMBID: row.RecordingMbid,
+			AudioFileID:    row.ID,
+			FilePath:       row.FilePath,
+			Title:          row.Title,
+			Artist:         row.ArtistName,
+			TrackNumber:    int(row.TrackNumber),
+			DiscNumber:     int(row.DiscNumber),
+			LengthMillis:   row.LengthMilliseconds,
+			RecordingMBID:  row.RecordingMbid,
+			AlbumTag:       row.AlbumName,
+			AlbumArtistTag: row.AlbumArtist,
 		})
 	}