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/http_cache.sql

@@ -1,5 +1,7 @@
 -- Short-lived HTTP response cache (search results, MB/LB lookups, etc).
 -- For long-lived enrichment data keyed by MBID, see artist_metadata.sql.
+
+
 CREATE TABLE IF NOT EXISTS http_cache (
     url_key     TEXT PRIMARY KEY,
     response    BLOB NOT NULL,
@@ -7,5 +9,7 @@ CREATE TABLE IF NOT EXISTS http_cache (
     entity_mbid TEXT NOT NULL DEFAULT '',
     entity_type TEXT NOT NULL DEFAULT ''
 );
+
 CREATE INDEX IF NOT EXISTS idx_http_cache_expires ON http_cache(expires_at);
+
 CREATE INDEX IF NOT EXISTS idx_http_cache_mbid ON http_cache(entity_mbid);