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/release_groups.sql

@@ -1,30 +1,20 @@
-CREATE TABLE IF NOT EXISTS release_groups (
+CREATE TABLE IF NOT EXISTS "release_groups" (
   id                     INTEGER PRIMARY KEY,
   name                   TEXT NOT NULL,
   cover_art_id           INTEGER,
   album_artist_credit_id INTEGER,
-  -- year is the *technical release year* of the album as it lives
-  -- in the user's library — typically the file's ID3 year tag,
-  -- which for remasters/reissues is the reissue year.
   year                   INTEGER,
-  -- original_year is the album's *first-release-date* year sourced
-  -- from MusicBrainz' release-group.first-release-date.  For a 2010
-  -- remaster of a 1973 album, year=2010 and original_year=1973.
-  -- Populated by autotag apply; NULL until the user accepts a
-  -- candidate (or for libraries that have never been autotagged).
-  -- Reads should COALESCE(original_year, year) to get the
-  -- preferred user-facing year.
-  original_year          INTEGER,
   total_tracks           INTEGER,
-  total_discs            INTEGER,
-  mbid                   TEXT,
+  total_discs            INTEGER, mbid TEXT, original_year INTEGER,
   FOREIGN KEY(cover_art_id) REFERENCES cover_art(id),
   FOREIGN KEY(album_artist_credit_id) REFERENCES artist_credit(id),
   UNIQUE(name, album_artist_credit_id)
-);
-
-CREATE INDEX IF NOT EXISTS idx_release_groups_cover_art_id
-    ON release_groups(cover_art_id);
+    );
 
 CREATE INDEX IF NOT EXISTS idx_release_groups_album_artist_credit_id
-    ON release_groups(album_artist_credit_id);
+  ON release_groups(album_artist_credit_id);
+
+CREATE INDEX IF NOT EXISTS idx_release_groups_cover_art_id
+  ON release_groups(cover_art_id);
+
+CREATE INDEX IF NOT EXISTS idx_release_groups_mbid ON release_groups(mbid) WHERE mbid IS NOT NULL;