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/database/sql/migrations/0001_tagging_items_synthetic.sql

@@ -0,0 +1,10 @@
+-- Adds SplitMixedFolder's synthetic-group bookkeeping to an
+-- existing tagging_items table.  A fresh database never runs this
+-- file: sql/schemas/tagging_items.sql already declares these
+-- columns, so applySchema's isFreshDatabase check stamps this
+-- version as applied without executing it.
+ALTER TABLE tagging_items ADD COLUMN synthetic INTEGER NOT NULL DEFAULT 0;
+ALTER TABLE tagging_items ADD COLUMN parent_group_key TEXT NOT NULL DEFAULT '';
+
+CREATE INDEX IF NOT EXISTS idx_tagging_items_parent_group_key
+    ON tagging_items(parent_group_key) WHERE parent_group_key != '';

backend/database/sql/migrations/0002_tagging_items_orphan_cleanup.sql

@@ -0,0 +1,24 @@
+-- Repairs tagging_items rows left behind by a library-scan bug: the
+-- rescan's orphan-cleanup phase deleted audio_files rows for files
+-- removed from disk without decrementing/clearing their tagging
+-- group, so a folder whose contents were fully replaced kept a
+-- phantom entry (stale track_count, no matching audio_files) in the
+-- autotag queue forever. The library scan code no longer has this
+-- gap, but a database written before the fix still carries the
+-- damage — this is a one-time repair, not ongoing bookkeeping.
+--
+-- Drop groups with no audio_files left at all.
+DELETE FROM tagging_items
+WHERE group_key NOT IN (
+  SELECT DISTINCT group_key FROM audio_files WHERE group_key != ''
+);
+
+-- Reconcile track_count for groups that are still alive but drifted
+-- (some, not all, of their tracks were removed without decrementing).
+UPDATE tagging_items
+SET track_count = (
+  SELECT COUNT(*) FROM audio_files WHERE audio_files.group_key = tagging_items.group_key
+)
+WHERE track_count != (
+  SELECT COUNT(*) FROM audio_files WHERE audio_files.group_key = tagging_items.group_key
+);