fix: 해구 통계 조회 에러 응답 노출 + 디버그 로깅 추가

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

src/main/java/gc/mda/signal_batch/monitoring/controller/MonitoringController.java

@@ -113,10 +113,14 @@ public class MonitoringController {
         """;
 
         try {
-            return queryJdbcTemplate.queryForList(sql);
+            List<Map<String, Object>> result = queryJdbcTemplate.queryForList(sql);
+            log.info("Haegu realtime query returned {} rows", result.size());
+            return result;
         } catch (Exception e) {
-            log.error("Failed to get realtime haegu status", e);
-            return new ArrayList<>();
+            log.error("Failed to get realtime haegu status: {}", e.getMessage(), e);
+            Map<String, Object> errorEntry = new HashMap<>();
+            errorEntry.put("error", e.getMessage());
+            return List.of(errorEntry);
         }
     }