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_wants.sql

@@ -0,0 +1,76 @@
+-- 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 download_wants (
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    mbid       TEXT    NOT NULL,
+    entity     TEXT    NOT NULL
+        CHECK(entity IN ('artist', 'release-group', 'release', 'recording')),
+    library_id INTEGER NOT NULL,
+
+    -- Display text, cached so the wanted list renders without touching
+    -- the explore index.  Neither is authoritative; the MBID is.
+    artist     TEXT    NOT NULL DEFAULT '',
+    title      TEXT    NOT NULL DEFAULT '',
+
+    scope      TEXT    NOT NULL DEFAULT 'future'
+        CHECK(scope IN ('future', 'all')),
+
+    -- secondary controls whether an artist want's expansion includes
+    -- compilations, live albums and remixes.  Off by default: someone
+    -- subscribing to an artist wants the albums, not six versions of
+    -- the same greatest-hits package.
+    secondary  INTEGER NOT NULL DEFAULT 0,
+
+    state      TEXT    NOT NULL DEFAULT 'wanted'
+        CHECK(state IN ('wanted', 'satisfied', 'paused')),
+
+    -- parent_id links a want the reconciler derived from an artist
+    -- want.  Deleting the artist takes its derived children with it,
+    -- but children the user pinned themselves have no parent and stay.
+    parent_id  INTEGER,
+
+    -- Retry bookkeeping.  attempts drives the backoff; last_error is
+    -- the most recent reason it did not work out, which for a wanted
+    -- item is information rather than a failure.
+    attempts     INTEGER  NOT NULL DEFAULT 0,
+    last_error   TEXT     NOT NULL DEFAULT '',
+    last_tried_at DATETIME,
+    next_try_at  DATETIME,
+
+    -- external_ids maps provider row ID to that provider's own
+    -- identifier for this want, for clients that keep their own
+    -- persistent list (a Lidarr artist ID).  JSON object.
+    external_ids TEXT    NOT NULL DEFAULT '{}',
+
+    created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    -- One want per thing per library.  Asking twice is not two wants,
+    -- and this is what lets an artist expansion re-run every reconcile
+    -- without accumulating duplicates.
+    UNIQUE(mbid, library_id),
+
+    FOREIGN KEY(library_id) REFERENCES libraries(id) ON DELETE CASCADE,
+    FOREIGN KEY(parent_id) REFERENCES download_wants(id) ON DELETE CASCADE
+);
+
+CREATE INDEX IF NOT EXISTS idx_download_wants_due
+    ON download_wants(next_try_at)
+    WHERE state = 'wanted';
+
+CREATE INDEX IF NOT EXISTS idx_download_wants_entity
+    ON download_wants(entity, state);
+
+CREATE INDEX IF NOT EXISTS idx_download_wants_parent
+    ON download_wants(parent_id);