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

frontend/src/store/library-store.ts

@@ -356,6 +356,25 @@ class LibraryStore {
         return libs;
     }
 
+    /**
+     * Resolves a library id to attach a download/want to: the selected
+     * filter if one is set, otherwise the first known library. Callers
+     * that need a library id (rather than "all libraries", which is
+     * what `getSelectedLibraryId()` returning null means for browsing)
+     * should use this instead of defaulting to 0 — id 0 never exists
+     * and trips the library_id foreign key on download_requests /
+     * download_wants.
+     */
+    async getDefaultLibraryId(): Promise<number | null> {
+        if (this.selectedLibraryIdValue !== null) {
+            return this.selectedLibraryIdValue;
+        }
+
+        const libraries = await this.getLibraries();
+
+        return libraries[0]?.id ?? null;
+    }
+
     // ===================================================================
     // SCROLL POSITION
     // ===================================================================