docs(documents): link density TODO to follow-up #481 (#385)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/document/DocumentService.java

@@ -141,10 +141,10 @@ public class DocumentService {
      * well under the 200ms p95 target. Cache-Control: max-age=300 on the
      * controller layer absorbs repeated browse loads.
      *
-     * <p>TODO: revisit when {@code documents > 50k} — at that scale move the
-     * aggregation into SQL (GROUP BY TO_CHAR(meta_date, 'YYYY-MM')) and accept
-     * that the criteria/specification surface needs a parallel native-query
-     * path. Surface this in the next perf audit.
+     * <p>Tracked in issue #481 for re-evaluation when {@code documents > 50k}
+     * — at that scale move the aggregation into SQL (GROUP BY TO_CHAR(meta_date,
+     * 'YYYY-MM')) and accept that the criteria/specification surface needs a
+     * parallel native-query path.
      */
     public DocumentDensityResult getDensity(
             String text, UUID sender, UUID receiver,