refactor(audit): add ROLLUP_ELIGIBLE constant to AuditKind

Single source of truth for the six kinds eligible for the activity rollup feed.

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

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

@@ -1,5 +1,7 @@
 package org.raddatz.familienarchiv.audit;
 
+import java.util.Set;
+
 public enum AuditKind {
 
     /** Payload: none */
@@ -24,5 +26,10 @@ public enum AuditKind {
     COMMENT_ADDED,
 
     /** Payload: {@code {"commentId": "uuid", "mentionedUserId": "uuid"}} */
-    MENTION_CREATED,
+    MENTION_CREATED;
+
+    public static final Set<AuditKind> ROLLUP_ELIGIBLE = Set.of(
+            TEXT_SAVED, FILE_UPLOADED, ANNOTATION_CREATED,
+            BLOCK_REVIEWED, COMMENT_ADDED, MENTION_CREATED
+    );
 }