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/libraries.sql

@@ -0,0 +1,20 @@
+-- One row per "go find me this", from the moment the user asks until
+-- the files are in the library or the attempt is abandoned.
+--
+-- release_mbid / release_group_mbid are the anchor: a request that
+-- carries one can be matched against a known tracklist at import time,
+-- which is what makes unattended completion safe.  Free-text requests
+-- (both NULL) are always presented to the user for confirmation.
+--
+-- `expected` caches the anchor's tracklist as JSON so ranking and
+-- import do not have to re-resolve it, and so a request survives the
+-- explore index being rebuilt underneath it.
+
+
+CREATE TABLE IF NOT EXISTS libraries (
+    id                     INTEGER PRIMARY KEY,
+    name                   TEXT NOT NULL,
+    path                   TEXT NOT NULL UNIQUE,
+    created_at             DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    autotag_warning_acked  INTEGER NOT NULL DEFAULT 0
+);