feat: data lifecycle rewrite, download clients, wanted list, and central catalog index

Ships the fresh-start schema cleanup: rebuilt explore catalog index
pipeline (dump import, artifact fetch/build, incremental listen-count
refresh), a new download subsystem (Lidarr/Prowlarr/qBittorrent/SABnzbd/
slskd/yt-dlp providers, staging, reconciliation, wanted list), and the
supporting schema/query/store changes across backend and frontend.

Also includes two smaller follow-ups: bump the central index's
rebuild-after cadence from 90 to 180 days, and remove the Explore
"library only" online/offline toggle entirely (frontend-only, no
backend counterpart) rather than carry unused UI/state.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2Agd9af5hE7qzti2ackiS

frontend/src/components/explore-view/explore-view.ts

@@ -869,10 +869,10 @@ export class ExploreView extends LitElement {
      * Wails call.  Called after search results are set.
      */
     /**
-     * Seed the thumbnail cache from local library data only.  Safe
-     * to call in library-only mode — does no API calls.  Reads from
-     * cachedAlbums (by MBID) and from any `_coverArt` underscore
-     * field that searchLibraryCache stamped on the release group.
+     * Seed the thumbnail cache from local library data only — does
+     * no API calls.  Reads from cachedAlbums (by MBID) and from any
+     * `_coverArt` underscore field that searchLibraryCache stamped
+     * on the release group.
      */
     private seedThumbnailsFromLibrary() {
         if (!this.results?.releaseGroups?.length) return;
@@ -906,8 +906,8 @@ export class ExploreView extends LitElement {
     }
 
     /**
-     * Seed the artist image cache from local library data only.
-     * Safe to call in library-only mode.  Reads from cachedArtists
+     * Seed the artist image cache from local library data only —
+     * does no API calls.  Reads from cachedArtists
      * by MBID and from any `_imageMedium`/`_imageSmall` underscore
      * field that searchLibraryCache stamped on the artist.  Falls
      * back to library album art when an artist has no portrait.