feat(sync): make annotation tombstone TTL configurable

The 30-day retention window for soft-deleted annotations was a package
const; move it behind the registry so it can be tuned live.

annotations.go:
- AnnotationService gains an optional *database.SettingsRegistry and a
  tombstoneTTL() helper. The skip-resurrect checks and the purge cutoff
  now call it instead of reading the TombstoneTTL const directly.
- Add ActiveTombstoneTTL() so callers outside the sync package can
  compute cutoffs consistently with the service.
- The package-level TombstoneTTL const is retained as the fallback for
  tests / unwired code paths.

kobo.go, koreader.go:
- The per-book tombstone sweep cutoff now uses
  h.annotationSvc.ActiveTombstoneTTL() instead of the wsync.TombstoneTTL
  const, so both the service and the handlers honor the configured TTL.

internal/handlers/koreader.go

@@ -854,7 +854,7 @@ func (h *KOReaderHandler) GetMetadata(c *echo.Context) error {
 		annotationsResponse.Bookmarks = append(annotationsResponse.Bookmarks, koreaderBookmark)
 	}
 
-	cutoff := pgtype.Timestamptz{Time: time.Now().Add(-wsync.TombstoneTTL), Valid: true}
+	cutoff := pgtype.Timestamptz{Time: time.Now().Add(-h.annotationSvc.ActiveTombstoneTTL()), Valid: true}
 	tombstones, _ := h.db.GetTombstonedAnnotationsForBook(c.Request().Context(), database.GetTombstonedAnnotationsForBookParams{
 		MediaItemID: pgBookUUID,
 		UserID:      pgUserID,