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/audio_files.sql

@@ -18,22 +18,28 @@ CREATE TABLE IF NOT EXISTS audio_files (
       'untagged', 'auto_matched', 'user_confirmed', 'user_skipped_permanent'
     )),
   group_key TEXT NOT NULL DEFAULT '',
+  -- File mtime as a Unix timestamp in seconds, captured at import.
+  -- Compared against the on-disk mtime during a scan to detect files
+  -- another application retagged in place.  0 means "never recorded"
+  -- (rows predating migration 47) and is treated as not-stale so an
+  -- upgrade does not re-import the whole library.
+  modified_at int NOT NULL DEFAULT 0,
   FOREIGN KEY(file_type_id) REFERENCES file_types(id),
   FOREIGN KEY(recording_id) REFERENCES recordings(id),
   FOREIGN KEY(library_id) REFERENCES libraries(id)
 );
 
+CREATE INDEX IF NOT EXISTS idx_audio_files_basename
+    ON audio_files(basename);
+
+CREATE INDEX IF NOT EXISTS idx_audio_files_group_key
+    ON audio_files(group_key) WHERE group_key != '';
+
+CREATE INDEX IF NOT EXISTS idx_audio_files_library_id
+  ON audio_files(library_id);
+
 CREATE INDEX IF NOT EXISTS idx_audio_files_recording_id
     ON audio_files(recording_id);
 
--- idx_audio_files_library_id is created by migration 6 (not here) because
--- on existing databases this schema file is a no-op (CREATE TABLE IF NOT EXISTS)
--- and the library_id column doesn't exist until the migration adds it.
---
--- idx_audio_files_tag_status_untagged + idx_audio_files_group_key are
--- created by migrations 31 and 32 for the same reason — on a pre-31
--- database the partial index predicates (`WHERE tag_status = '...'`
--- and `WHERE group_key != ''`) would reference columns that don't
--- yet exist, since CREATE TABLE IF NOT EXISTS does not add columns
--- to existing tables.  sqlc still sees the columns above, and fresh
--- DBs pick up the indexes inside the migrations.
+CREATE INDEX IF NOT EXISTS idx_audio_files_tag_status_untagged
+    ON audio_files(library_id) WHERE tag_status = 'untagged';