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/lyrics_index.sql

@@ -7,6 +7,8 @@
 -- tokenised inverted index, so it stays compact even for large
 -- libraries.  contentless_delete=1 lets us delete/reinsert a single
 -- row when a track's lyrics change (scan update or LRCLIB backfill).
+
+
 CREATE VIRTUAL TABLE IF NOT EXISTS lyrics_index USING fts5(
     lyrics,
     content='',