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/datamap/datamap.go

@@ -268,6 +268,13 @@ var tables = []Table{
 		Name: "release_to_rg", Kind: Cache, Lifetime: Retained,
 		Note: "Release to release-group mapping from the dump.",
 	},
+	{
+		Name: "schema_migrations", Kind: Derived, Lifetime: Retained,
+		Note: "Bookkeeping for sql/migrations: which numbered files have " +
+			"run. Safe to lose — replaying an already-applied migration " +
+			"tolerates its ALTER TABLE ADD COLUMN as a no-op and just " +
+			"re-records it.",
+	},
 	{
 		Name: "search_clicks", Kind: Authored, Lifetime: Retained,
 		Note: "Which results the user picked, used to rank future " +