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/events.ts

@@ -69,6 +69,9 @@ export const Events = {
     ArtistDiscographyReady: "ArtistDiscographyReady",
     ArtistSimilarReady: "ArtistSimilarReady",
     AlbumReleasesReady: "AlbumReleasesReady",
+    DownloadProvidersChanged: "DownloadProvidersChanged",
+    DownloadsChanged: "DownloadsChanged",
+    WantedListChanged: "WantedListChanged",
 } as const;
 
 export type EventName = (typeof Events)[keyof typeof Events];