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/dumpcatalog.go

@@ -1,3 +1,5 @@
+//go:build indexbuild
+
 package explore
 
 import (
@@ -440,7 +442,7 @@ func (a *artistTopRG) add(rg uuid16, listens uint32) {
 func (imp *dumpImporter) scanCanonicalDump(
 	ctx context.Context, url string, ks *keptSets,
 ) (*canonicalScan, error) {
-	stream := newResumableReader(ctx, imp.httpClient, url, 0)
+	stream := imp.openDumpStream(ctx, url, 0)
 
 	defer func() { _ = stream.Close() }()