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/autotag/distance.go

@@ -192,6 +192,16 @@ func rotateEndWord(s string) string {
 	return s
 }
 
+// TitleSimilarity exposes titleSimilarity for callers outside the
+// package that compare music metadata strings and should get the same
+// answer the tagger would.  The download pipeline uses it to match
+// candidate filenames against an expected tracklist — Soulseek and
+// torrent results carry paths, not tags, so filename comparison is the
+// only signal available before the bytes arrive.
+func TitleSimilarity(a, b string) float64 {
+	return titleSimilarity(a, b)
+}
+
 // titleSimilarity returns a score in [0, 1] from stringDist.  1.0
 // means identical after normalization, 0.0 means fully dissimilar.
 func titleSimilarity(a, b string) float64 {