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/download_providers.sql

@@ -0,0 +1,28 @@
+-- Download clients the user has connected: an slskd daemon, a Lidarr
+-- instance, yt-dlp on PATH.  One row per configured instance, so two
+-- Prowlarr servers or two Soulseek accounts coexist.
+--
+-- Secrets (API keys, passwords) are NOT stored here.  They live in a
+-- 0600 file keyed by this row's id, so this table can be dumped into a
+-- bug report without redaction.  `settings` holds only non-sensitive
+-- values (host, port, category, output format) as a JSON object.
+--
+-- `kind` names the adapter implementation and is looked up in the
+-- provider registry at startup; a row whose kind no longer exists is
+-- reported to the user rather than silently dropped.
+
+
+CREATE TABLE IF NOT EXISTS download_providers (
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    kind       TEXT    NOT NULL,
+    name       TEXT    NOT NULL,
+    enabled    INTEGER NOT NULL DEFAULT 1,
+    -- priority breaks ties between providers that found equally good
+    -- candidates.  Higher wins; 50 is the neutral default.
+    priority   INTEGER NOT NULL DEFAULT 50,
+    settings   TEXT    NOT NULL DEFAULT '{}',
+    created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX IF NOT EXISTS idx_download_providers_enabled
+    ON download_providers(enabled);