fix: history/detail API 500 오류 — candidate_count 컬럼 부재 시 안전 처리

mapGroupRow에서 candidate_count를 읽을 때 optionalInt로 변경하여
해당 컬럼이 없는 SQL (history, detail)에서도 정상 동작하도록 수정

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -1409,7 +1409,7 @@ public class GroupPolygonService {
                 .members(members)
                 .color(rs.getString("color"))
                 .resolution(rs.getString("resolution"))
-                .candidateCount(nullableInt(rs, "candidate_count"))
+                .candidateCount(optionalInt(rs, "candidate_count"))
                 .parentInference(mapParentInferenceSummary(rs))
                 .build();
     }
@@ -1566,6 +1566,15 @@ public class GroupPolygonService {
         return ((Number) value).intValue();
     }
 
+    /** 컬럼이 ResultSet에 존재하지 않으면 null 반환 (history/detail SQL 호환) */
+    private Integer optionalInt(ResultSet rs, String column) throws SQLException {
+        try {
+            return nullableInt(rs, column);
+        } catch (SQLException e) {
+            return null;
+        }
+    }
+
     private Map<String, Object> parseJsonObject(String json) {
         return parseJsonValue(json, new TypeReference<Map<String, Object>>() {}, Map.of());
     }