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/library/coverart.go

@@ -48,6 +48,32 @@ var thumbnailTiers = []thumbnailTier{
 // multi-tier system.  Kept for migration purposes only.
 const legacyThumbSuffix = "_thumb"
 
+// CoverArtFileSet returns every file on disk belonging to one cover art
+// entry: the original plus each generated size variant, plus the legacy
+// _thumb file for databases that predate the multi-tier thumbnails.
+//
+// Only the original is recorded in cover_art.file_path — the variants
+// are derived filenames — so any code deleting cover art has to expand
+// the set or the thumbnails are orphaned.
+func CoverArtFileSet(originalPath string) []string {
+	dir := filepath.Dir(originalPath)
+	base := filepath.Base(originalPath)
+
+	paths := make([]string, 0, len(thumbnailTiers)+2) //nolint:mnd
+
+	paths = append(paths, originalPath)
+
+	for _, tier := range thumbnailTiers {
+		paths = append(paths, filepath.Join(
+			dir, coverart.SizedFilename(base, tier.Suffix),
+		))
+	}
+
+	return append(paths, filepath.Join(
+		dir, coverart.SizedFilename(base, legacyThumbSuffix),
+	))
+}
+
 // isSizedVariant reports whether a filename contains any known size suffix
 // (current tiers or legacy).
 func isSizedVariant(name string) bool {