feat(scanner): multifile folder/zip sidecar detection (faceted plan Phase 6)

Reverses the Phase 4 scope cut: for EXISTING folder/zip audiobooks a
sidecar edit was invisible to everything, including force rescan —
updateAudioBookItem writes only scan-derived fields, and these formats
often have no useful embedded tags, so the sidecar is the only source.

- Change fingerprint gains the manifest compare in both processors:
  changed := size differs || track payload differs || manifest drifted.
  The ladder runs against the bulk-loaded map entry (stat pairs only,
  zero extra queries); NULL stored manifest backfills without
  re-extraction (deploy-day rule, same as single-file).
- Manifest-changed applies the once-extracted book metadata to the
  editorial columns (updateMediaItemWithMetadata, overrides protected)
  and re-runs the playlist hook. The apply runs BEFORE the focused
  payload write: the generic update's format-group pass derives mime
  from the sentinel (none) and chapters from metadata.ChapterCount
  (unset) — the payload write must win (rescanMultiFileAudioBook's
  ordering). Track-only changes keep the existing editorial-keep
  update; force rescan keeps it too.
- Manifests seeded at multifile create (folder names derive from the
  sentinel — the exact representative the extractor reads — via
  detectSidecarChangeWithRepresentative, so manifest and extractor can
  never disagree; a directory-basename cover is ignored by the
  extractor and therefore not tracked).
- Zip parity fixes: the fast path now lifts archive state like the
  folder path always did (a returned zip book stayed archived
  forever), and the change branch handles the unchanged-archived case.
- Single-file sidecar change path: extract-failure guard — the old
  non-nil empty &MediaMetadata{} fallback bypassed the update path's
  filename-title guard (preExtracted == nil only) and would blank the
  title on a failed extraction; the row is now left untouched and the
  manifest refreshed. Empty-success extraction (sidecar removed from a
  tag-less book) now passes nil so the create-equivalent chain (tags →
  filename) re-derives instead of blanking.
- Multifile changes now count in the scan summary (changedItems);
  they were previously uncounted.

Tests: sentinel-name derivation unit pin; folder sidecar edit
(title + collection sync + fresh manifest + steady-state skip),
track-only change keeps editorial, zip sidecar edit + archive
restore, sidecar removal falls back to filename (the wipe-guard's
testable sibling). Full container suite: green except the documented
pre-existing TestAdminCreateUser red.

internal/services/media_scanner_manifest.go

@@ -228,7 +228,18 @@ func (s *MediaScanner) storeManifest(ctx context.Context, libraryID pgtype.UUID,
 // manifest after a change-path update.
 func (s *MediaScanner) detectSidecarChange(ctx context.Context, entry *scanEntry, libraryID pgtype.UUID, path string) bool {
 	dir := bookSidecarDir(path)
-	computed := statSidecarManifest(path, dir)
+	return s.detectSidecarChangeWithRepresentative(ctx, entry, libraryID, path, dir)
+}
+
+// detectSidecarChangeWithRepresentative is detectSidecarChange with the
+// sidecar-name derivation decoupled from the map key. Multi-file books
+// (faceted plan Phase 6) are keyed by the item's file_path (the folder
+// root / zip), but the extractor reads a folder book's sidecars through
+// the sentinel INSIDE the root — whose basename yields no "<base>.<ext>"
+// cover candidates — so the manifest must stat the representative's name
+// list to never disagree with what extraction consumes.
+func (s *MediaScanner) detectSidecarChangeWithRepresentative(ctx context.Context, entry *scanEntry, libraryID pgtype.UUID, representative, dir string) bool {
+	computed := statSidecarManifest(representative, dir)
 	stored := decodeSidecarManifest(entry.Manifest)
 
 	if len(computed.Entries) == 0 {
@@ -240,7 +251,7 @@ func (s *MediaScanner) detectSidecarChange(ctx context.Context, entry *scanEntry
 
 	if stored == nil {
 		// Deploy-day backfill: hash + store WITHOUT re-extraction.
-		s.storeManifest(ctx, libraryID, entry, hashSidecarManifest(path, dir, computed))
+		s.storeManifest(ctx, libraryID, entry, hashSidecarManifest(representative, dir, computed))
 		return false
 	}