feat(backend): 그룹 폴리곤 API — 목록/상세/히스토리 엔드포인트

- GroupPolygonController: GET /api/vessel-analysis/groups (목록, 상세, 히스토리)
- GroupPolygonService: JdbcTemplate + ST_AsGeoJSON + Caffeine 5분 캐시
- GroupPolygonDto: GeoJSON polygon + members JSONB 응답 구조
- CacheConfig: GROUP_POLYGONS 캐시 키 추가

backend/src/main/java/gc/mda/kcg/config/CacheConfig.java

@@ -21,6 +21,7 @@ public class CacheConfig {
     public static final String SEISMIC = "seismic";
     public static final String PRESSURE = "pressure";
     public static final String VESSEL_ANALYSIS = "vessel-analysis";
+    public static final String GROUP_POLYGONS = "group-polygons";
 
     @Bean
     public CacheManager cacheManager() {
@@ -29,7 +30,8 @@ public class CacheConfig {
                 OSINT_IRAN, OSINT_KOREA,
                 SATELLITES,
                 SEISMIC, PRESSURE,
-                VESSEL_ANALYSIS
+                VESSEL_ANALYSIS,
+                GROUP_POLYGONS
         );
         manager.setCaffeine(Caffeine.newBuilder()
                 .expireAfterWrite(2, TimeUnit.DAYS)

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

@@ -0,0 +1,51 @@
+package gc.mda.kcg.domain.fleet;
+
+import lombok.RequiredArgsConstructor;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.*;
+
+import java.util.List;
+import java.util.Map;
+
+@RestController
+@RequestMapping("/api/vessel-analysis/groups")
+@RequiredArgsConstructor
+public class GroupPolygonController {
+
+    private final GroupPolygonService groupPolygonService;
+
+    /**
+     * 전체 그룹 폴리곤 목록 (최신 스냅샷, 5분 캐시)
+     */
+    @GetMapping
+    public ResponseEntity<Map<String, Object>> getGroups() {
+        List<GroupPolygonDto> groups = groupPolygonService.getLatestGroups();
+        return ResponseEntity.ok(Map.of(
+                "count", groups.size(),
+                "items", groups
+        ));
+    }
+
+    /**
+     * 특정 그룹 상세 (멤버, 면적, 폴리곤 생성 근거)
+     */
+    @GetMapping("/{groupKey}/detail")
+    public ResponseEntity<GroupPolygonDto> getGroupDetail(@PathVariable String groupKey) {
+        GroupPolygonDto detail = groupPolygonService.getGroupDetail(groupKey);
+        if (detail == null) {
+            return ResponseEntity.notFound().build();
+        }
+        return ResponseEntity.ok(detail);
+    }
+
+    /**
+     * 특정 그룹 히스토리 (시간별 폴리곤 변화)
+     */
+    @GetMapping("/{groupKey}/history")
+    public ResponseEntity<List<GroupPolygonDto>> getGroupHistory(
+            @PathVariable String groupKey,
+            @RequestParam(defaultValue = "24") int hours) {
+        List<GroupPolygonDto> history = groupPolygonService.getGroupHistory(groupKey, hours);
+        return ResponseEntity.ok(history);
+    }
+}

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

@@ -0,0 +1,27 @@
+package gc.mda.kcg.domain.fleet;
+
+import com.fasterxml.jackson.annotation.JsonInclude;
+import lombok.Builder;
+import lombok.Getter;
+
+import java.util.List;
+import java.util.Map;
+
+@Getter
+@Builder
+@JsonInclude(JsonInclude.Include.NON_NULL)
+public class GroupPolygonDto {
+    private String groupType;
+    private String groupKey;
+    private String groupLabel;
+    private String snapshotTime;
+    private Object polygon;       // GeoJSON Polygon (parsed from ST_AsGeoJSON)
+    private double centerLat;
+    private double centerLon;
+    private double areaSqNm;
+    private int memberCount;
+    private String zoneId;
+    private String zoneName;
+    private List<Map<String, Object>> members;
+    private String color;
+}

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

@@ -0,0 +1,131 @@
+package gc.mda.kcg.domain.fleet;
+
+import com.fasterxml.jackson.core.type.TypeReference;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import gc.mda.kcg.config.CacheConfig;
+import lombok.RequiredArgsConstructor;
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.cache.Cache;
+import org.springframework.cache.CacheManager;
+import org.springframework.jdbc.core.JdbcTemplate;
+import org.springframework.stereotype.Service;
+
+import java.sql.ResultSet;
+import java.sql.SQLException;
+import java.util.List;
+import java.util.Map;
+
+@Slf4j
+@Service
+@RequiredArgsConstructor
+public class GroupPolygonService {
+
+    private final JdbcTemplate jdbcTemplate;
+    private final CacheManager cacheManager;
+    private final ObjectMapper objectMapper;
+
+    private static final String LATEST_GROUPS_SQL = """
+            SELECT group_type, group_key, group_label, snapshot_time,
+                   ST_AsGeoJSON(polygon) AS polygon_geojson,
+                   ST_Y(center_point) AS center_lat, ST_X(center_point) AS center_lon,
+                   area_sq_nm, member_count, zone_id, zone_name, members, color
+            FROM kcg.group_polygon_snapshots
+            WHERE snapshot_time = (SELECT MAX(snapshot_time) FROM kcg.group_polygon_snapshots)
+            ORDER BY group_type, member_count DESC
+            """;
+
+    private static final String GROUP_DETAIL_SQL = """
+            SELECT group_type, group_key, group_label, snapshot_time,
+                   ST_AsGeoJSON(polygon) AS polygon_geojson,
+                   ST_Y(center_point) AS center_lat, ST_X(center_point) AS center_lon,
+                   area_sq_nm, member_count, zone_id, zone_name, members, color
+            FROM kcg.group_polygon_snapshots
+            WHERE group_key = ?
+            ORDER BY snapshot_time DESC
+            LIMIT 1
+            """;
+
+    private static final String GROUP_HISTORY_SQL = """
+            SELECT group_type, group_key, group_label, snapshot_time,
+                   ST_AsGeoJSON(polygon) AS polygon_geojson,
+                   ST_Y(center_point) AS center_lat, ST_X(center_point) AS center_lon,
+                   area_sq_nm, member_count, zone_id, zone_name, members, color
+            FROM kcg.group_polygon_snapshots
+            WHERE group_key = ? AND snapshot_time > NOW() - CAST(? || ' hours' AS INTERVAL)
+            ORDER BY snapshot_time DESC
+            """;
+
+    /**
+     * 최신 스냅샷의 전체 그룹 폴리곤 목록 (5분 캐시).
+     */
+    @SuppressWarnings("unchecked")
+    public List<GroupPolygonDto> getLatestGroups() {
+        Cache cache = cacheManager.getCache(CacheConfig.GROUP_POLYGONS);
+        if (cache != null) {
+            Cache.ValueWrapper wrapper = cache.get("data");
+            if (wrapper != null) {
+                return (List<GroupPolygonDto>) wrapper.get();
+            }
+        }
+
+        List<GroupPolygonDto> results = jdbcTemplate.query(LATEST_GROUPS_SQL, this::mapRow);
+
+        if (cache != null) {
+            cache.put("data", results);
+        }
+        return results;
+    }
+
+    /**
+     * 특정 그룹의 최신 상세 정보.
+     */
+    public GroupPolygonDto getGroupDetail(String groupKey) {
+        List<GroupPolygonDto> results = jdbcTemplate.query(GROUP_DETAIL_SQL, this::mapRow, groupKey);
+        return results.isEmpty() ? null : results.get(0);
+    }
+
+    /**
+     * 특정 그룹의 시간별 히스토리.
+     */
+    public List<GroupPolygonDto> getGroupHistory(String groupKey, int hours) {
+        return jdbcTemplate.query(GROUP_HISTORY_SQL, this::mapRow, groupKey, String.valueOf(hours));
+    }
+
+    private GroupPolygonDto mapRow(ResultSet rs, int rowNum) throws SQLException {
+        Object polygonObj = null;
+        String polygonJson = rs.getString("polygon_geojson");
+        if (polygonJson != null) {
+            try {
+                polygonObj = objectMapper.readValue(polygonJson, new TypeReference<Map<String, Object>>() {});
+            } catch (Exception e) {
+                log.warn("Failed to parse polygon GeoJSON: {}", e.getMessage());
+            }
+        }
+
+        List<Map<String, Object>> members = List.of();
+        String membersJson = rs.getString("members");
+        if (membersJson != null) {
+            try {
+                members = objectMapper.readValue(membersJson, new TypeReference<>() {});
+            } catch (Exception e) {
+                log.warn("Failed to parse members JSON: {}", e.getMessage());
+            }
+        }
+
+        return GroupPolygonDto.builder()
+                .groupType(rs.getString("group_type"))
+                .groupKey(rs.getString("group_key"))
+                .groupLabel(rs.getString("group_label"))
+                .snapshotTime(rs.getString("snapshot_time"))
+                .polygon(polygonObj)
+                .centerLat(rs.getDouble("center_lat"))
+                .centerLon(rs.getDouble("center_lon"))
+                .areaSqNm(rs.getDouble("area_sq_nm"))
+                .memberCount(rs.getInt("member_count"))
+                .zoneId(rs.getString("zone_id"))
+                .zoneName(rs.getString("zone_name"))
+                .members(members)
+                .color(rs.getString("color"))
+                .build();
+    }
+}