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/database/sql/schemas/artist_metadata.sql

@@ -2,6 +2,8 @@
 -- Sources: audiodb, fanart, wikidata-p18, wikipedia-lead, mb:artist-rels.
 -- No TTL — this data changes very rarely and is the backing store for
 -- the artist detail page.
+
+
 CREATE TABLE IF NOT EXISTS artist_metadata (
     mbid       TEXT NOT NULL,
     source     TEXT NOT NULL,
@@ -9,4 +11,5 @@ CREATE TABLE IF NOT EXISTS artist_metadata (
     fetched_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     PRIMARY KEY (mbid, source)
 );
+
 CREATE INDEX IF NOT EXISTS idx_artist_metadata_mbid ON artist_metadata(mbid);