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/artistimage.go

@@ -36,7 +36,7 @@ const (
 	artistImageTimeout      = 10 * time.Second
 	artistImageCacheTTL     = 365 * 24 * time.Hour // positive results: ~permanent
 	artistImageMissCacheTTL = 30 * 24 * time.Hour  // negative results: retry monthly
-	artistImageBaseDir      = "artist-images"
+	artistImageBaseDir      = ArtistImageDirName
 	artistImageMaxBytes     = 2 * 1024 * 1024
 	artistImageMaxSize      = 500 // max dimension for stored full-res images
 	maxImagesPerArtist      = 10