feat: Implement entity join functionality in V2Repeater and configuration panel

- Added support for entity joins in the V2Repeater component, allowing for automatic resolution of foreign key references to display data from related tables.
- Introduced a new `resolveEntityJoins` function to handle the fetching and mapping of reference data based on configured entity joins.
- Enhanced the V2RepeaterConfigPanel to manage entity join configurations, including loading available columns and toggling join settings.
- Updated the data handling logic to incorporate mapping rules for incoming data, ensuring that only necessary fields are retained during processing.
- Improved user experience by providing clear logging and feedback during entity join resolution and data mapping operations.

frontend/lib/registry/components/modal-repeater-table/ItemSelectionModal.tsx

@@ -305,12 +305,26 @@ export function ItemSelectionModal({
     onOpenChange(false);
   };
 
-  // 이미 추가된 항목인지 확인
+  // 이미 추가된 항목인지 확인 (매핑된 데이터의 _sourceData도 검사)
   const isAlreadyAdded = (item: any): boolean => {
-    if (!uniqueField) return false;
-    return alreadySelected.some(
-      (selected) => selected[uniqueField] === item[uniqueField]
-    );
+    const checkField = uniqueField || "id";
+    const itemValue = item[checkField];
+    if (itemValue === undefined || itemValue === null) return false;
+    const strItemValue = String(itemValue);
+
+    return alreadySelected.some((selected) => {
+      // _sourceData 우선 확인 (DB 로드 항목의 참조 ID가 매핑되어 있음)
+      const sourceValue = selected._sourceData?.[checkField];
+      if (sourceValue !== undefined && sourceValue !== null && String(sourceValue) === strItemValue) {
+        return true;
+      }
+      // _sourceData에 없으면 직접 필드 비교 (동일 필드명인 경우)
+      const directValue = selected[checkField];
+      if (directValue !== undefined && directValue !== null && String(directValue) === strItemValue) {
+        return true;
+      }
+      return false;
+    });
   };
 
   // 이미 추가된 항목 제외한 결과 필터링