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

backend/explore/coverartproxy.go

@@ -23,7 +23,7 @@ var ErrCoverArt = errors.New("cover art fetch failed")
 const (
 	// thumbnailDir is the subdirectory under the user data dir
 	// where cached cover art thumbnails are stored.
-	thumbnailDir = "cover-art-cache"
+	thumbnailDir = CoverArtCacheDirName
 
 	// thumbnailTimeout is the HTTP timeout for fetching a thumbnail.
 	thumbnailTimeout = 10 * time.Second