feat: Item Master 하이브리드 구조 전환 및 독립 API 추가

- CASCADE FK → 독립 엔티티 + entity_relationships 링크 테이블
- 독립 API 10개 추가 (섹션/필드/BOM CRUD, clone, usage)
- SectionTemplate 모델 제거 → ItemSection.is_template 통합
- 페이지-섹션, 섹션-필드, 섹션-BOM 링크/언링크 API 14개 추가
- Swagger 문서 업데이트

app/Http/Controllers/Api/V1/ItemMaster/ItemSectionController.php

@@ -4,17 +4,71 @@
 
 use App\Helpers\ApiResponse;
 use App\Http\Controllers\Controller;
+use App\Http\Requests\ItemMaster\IndependentSectionStoreRequest;
 use App\Http\Requests\ItemMaster\ItemSectionStoreRequest;
 use App\Http\Requests\ItemMaster\ItemSectionUpdateRequest;
 use App\Http\Requests\ItemMaster\ReorderRequest;
 use App\Services\ItemMaster\ItemSectionService;
+use Illuminate\Http\Request;
 
 class ItemSectionController extends Controller
 {
     public function __construct(private ItemSectionService $service) {}
 
     /**
-     * 섹션 생성
+     * 독립 섹션 목록 조회
+     *
+     * GET /api/v1/item-master/sections
+     */
+    public function index(Request $request)
+    {
+        return ApiResponse::handle(function () use ($request) {
+            $isTemplate = $request->has('is_template')
+                ? filter_var($request->query('is_template'), FILTER_VALIDATE_BOOLEAN, FILTER_NULL_ON_FAILURE)
+                : null;
+
+            return $this->service->index($isTemplate);
+        }, __('message.fetched'));
+    }
+
+    /**
+     * 독립 섹션 생성 (페이지 연결 없음)
+     *
+     * POST /api/v1/item-master/sections
+     */
+    public function storeIndependent(IndependentSectionStoreRequest $request)
+    {
+        return ApiResponse::handle(function () use ($request) {
+            return $this->service->storeIndependent($request->validated());
+        }, __('message.created'));
+    }
+
+    /**
+     * 섹션 복제
+     *
+     * POST /api/v1/item-master/sections/{id}/clone
+     */
+    public function clone(int $id)
+    {
+        return ApiResponse::handle(function () use ($id) {
+            return $this->service->clone($id);
+        }, __('message.created'));
+    }
+
+    /**
+     * 섹션 사용처 조회
+     *
+     * GET /api/v1/item-master/sections/{id}/usage
+     */
+    public function getUsage(int $id)
+    {
+        return ApiResponse::handle(function () use ($id) {
+            return $this->service->getUsage($id);
+        }, __('message.fetched'));
+    }
+
+    /**
+     * 섹션 생성 (페이지에 연결)
      *
      * POST /api/v1/item-master/pages/{pageId}/sections
      */