fix(backend): 그룹 폴리곤 캐시 TTL 5분 제한

- Caffeine 전역 TTL(2일) 대신 서비스 내 수동 5분 TTL 체크
- 5분마다 DB에서 최신 스냅샷 재조회 보장

backend/src/main/java/gc/mda/kcg/domain/fleet/GroupPolygonService.java

@@ -24,6 +24,9 @@ public class GroupPolygonService {
     private final CacheManager cacheManager;
     private final ObjectMapper objectMapper;
 
+    private static final long CACHE_TTL_MS = 5 * 60_000L; // 5분
+    private volatile long lastCacheTime = 0;
+
     private static final String LATEST_GROUPS_SQL = """
             SELECT group_type, group_key, group_label, snapshot_time,
                    ST_AsGeoJSON(polygon) AS polygon_geojson,
@@ -61,7 +64,9 @@ public class GroupPolygonService {
     @SuppressWarnings("unchecked")
     public List<GroupPolygonDto> getLatestGroups() {
         Cache cache = cacheManager.getCache(CacheConfig.GROUP_POLYGONS);
-        if (cache != null) {
+        long now = System.currentTimeMillis();
+
+        if (cache != null && (now - lastCacheTime) < CACHE_TTL_MS) {
             Cache.ValueWrapper wrapper = cache.get("data");
             if (wrapper != null) {
                 return (List<GroupPolygonDto>) wrapper.get();
@@ -72,6 +77,7 @@ public class GroupPolygonService {
 
         if (cache != null) {
             cache.put("data", results);
+            lastCacheTime = now;
         }
         return results;
     }