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/recommend.go

@@ -59,9 +59,16 @@ func Recommend(g Group, candidates []Candidate) Recommendation {
 
 	// Cap: missing or unmatched tracks mean the alignment itself is
 	// incomplete, however good the matched tracks look (beets caps
-	// these penalties at "medium" the same way).
+	// these penalties at "medium" the same way).  A synthetic
+	// (tag-clustered) group is, by construction, a subset of a
+	// bigger folder, so AlignmentMissing (the candidate has tracks
+	// the group doesn't) is the expected shape rather than a defect
+	// and doesn't cap the recommendation.  AlignmentUnmatched (the
+	// group has a track the candidate doesn't) is still a real
+	// discrepancy regardless of source.
 	for _, a := range top.Alignments {
-		if a.Status == AlignmentMissing || a.Status == AlignmentUnmatched {
+		if a.Status == AlignmentUnmatched ||
+			(a.Status == AlignmentMissing && !g.Synthetic) {
 			rec = minRecommendation(rec, RecommendationMedium)
 
 			break