refactor(test): use ObjectMapper for payload JSON in audit test helper

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

backend/src/test/java/org/raddatz/familienarchiv/dashboard/AuditLogQueryRepositoryRolledUpTest.java

@@ -14,6 +14,9 @@ import org.springframework.jdbc.core.namedparam.MapSqlParameterSource;
 import org.springframework.jdbc.core.namedparam.NamedParameterJdbcTemplate;
 import org.springframework.transaction.annotation.Transactional;
 
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.ObjectMapper;
+
 import java.time.Instant;
 import java.time.OffsetDateTime;
 import java.util.List;
@@ -33,6 +36,8 @@ class AuditLogQueryRepositoryRolledUpTest {
     static final UUID DOC_ID = UUID.fromString("eeeeeeee-eeee-eeee-eeee-eeeeeeeeeeee");
     static final UUID OTHER_DOC_ID = UUID.fromString("ffffffff-ffff-ffff-ffff-ffffffffffff");
 
+    private static final ObjectMapper MAPPER = new ObjectMapper();
+
     @Autowired AuditLogQueryRepository auditLogQueryRepository;
     @Autowired JdbcTemplate jdbcTemplate;
 
@@ -60,10 +65,12 @@ class AuditLogQueryRepositoryRolledUpTest {
     }
 
     private void insertAuditEvent(UUID actorId, UUID docId, String kind, Instant happenedAt, Map<String, String> payload) {
-        String payloadJson = payload.isEmpty() ? null
-                : payload.entrySet().stream()
-                    .map(e -> "\"" + e.getKey() + "\": \"" + e.getValue() + "\"")
-                    .collect(java.util.stream.Collectors.joining(", ", "{", "}"));
+        String payloadJson;
+        try {
+            payloadJson = payload.isEmpty() ? null : MAPPER.writeValueAsString(payload);
+        } catch (JsonProcessingException e) {
+            throw new RuntimeException(e);
+        }
         MapSqlParameterSource params = new MapSqlParameterSource()
                 .addValue("kind", kind)
                 .addValue("actor", actorId)