fix: 해구 통계 ROUND 함수 타입 캐스팅 오류 수정

GREATEST() 반환값이 double precision이라 ROUND(dp, int) 호출 시
PostgreSQL에서 미지원 함수 에러 발생 → 전체 나눗셈 결과를 ::numeric 캐스팅

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

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

@@ -93,8 +93,8 @@ public class MonitoringController {
                 CONCAT('대해구 ', h.haegu_no) as haegu_name,
                 COUNT(DISTINCT a.mmsi) as current_vessels,
                 ROUND(AVG(a.sog)::numeric, 1) as avg_speed,
-                ROUND(COUNT(DISTINCT a.mmsi)::numeric /
+                ROUND((COUNT(DISTINCT a.mmsi)::numeric /
-                    GREATEST((h.max_lat - h.min_lat) * (h.max_lon - h.min_lon) * 12321, 0.01),
+                    GREATEST((h.max_lat - h.min_lat) * (h.max_lon - h.min_lon) * 12321, 0.01))::numeric,
                     4) as avg_density,
                 MAX(a.last_update) as last_update,
                 h.center_lon,