test(audit): add youParticipated and youMentioned repository tests

Add isYouParticipated() to ActivityFeedRow interface and cover four
behaviours in AuditLogQueryRepositoryRolledUpTest: youParticipated
true/false and retroactive youMentioned true/false coverage.

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

backend/src/main/java/org/raddatz/familienarchiv/audit/ActivityFeedRow.java

@@ -12,6 +12,7 @@ public interface ActivityFeedRow {
     UUID getDocumentId();
     Instant getHappenedAt();
     boolean isYouMentioned();
+    boolean isYouParticipated();
     int getCount();
     Instant getHappenedAtUntil();
 }