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/similar_artist_map.sql

@@ -0,0 +1,10 @@
+CREATE TABLE IF NOT EXISTS similar_artist_map (
+  source_artist_mbid  TEXT NOT NULL,
+  similar_artist_mbid TEXT NOT NULL,
+  similar_artist_name TEXT NOT NULL,
+  score               INTEGER NOT NULL DEFAULT 0,
+  PRIMARY KEY (source_artist_mbid, similar_artist_mbid)
+    );
+
+CREATE INDEX IF NOT EXISTS idx_similar_artist_map_source
+    ON similar_artist_map(source_artist_mbid);