docs(audit): correct DOCUMENT_DELETED payload javadoc to none (#805)

The call site passes a null payload and carries the id in the documentId
column (matching FILE_UPLOADED), so the javadoc claiming Payload:
{"documentId": "uuid"} misdescribed the audit schema. Audit javadocs are the
contract forensic queries are written against.

Addresses @felix, @nora and @elicit review on PR #806.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -54,7 +54,7 @@ public enum AuditKind {
 
     // --- Documents ---
 
-    /** Payload: {@code {"documentId": "uuid"}} */
+    /** Payload: none — the deleted document's id is carried in the documentId column */
     DOCUMENT_DELETED,
 
     // --- Reading Journeys (Lesereisen) ---