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_images.sql

@@ -0,0 +1,19 @@
+CREATE TABLE IF NOT EXISTS artist_images (
+  id          INTEGER PRIMARY KEY AUTOINCREMENT,
+  artist_mbid TEXT NOT NULL,
+  source      TEXT NOT NULL,
+  source_url  TEXT NOT NULL,
+  file_path   TEXT NOT NULL,
+  is_primary  INTEGER NOT NULL DEFAULT 0,
+  sort_order  INTEGER NOT NULL DEFAULT 0,
+  width       INTEGER,
+  height      INTEGER,
+  file_size   INTEGER,
+  created_at  DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+    );
+
+CREATE INDEX IF NOT EXISTS idx_artist_images_mbid
+    ON artist_images(artist_mbid);
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_artist_images_source
+    ON artist_images(artist_mbid, source, source_url);