v0.0.19 | Internet Gateway / Routing Table 관리 API 추가

- ApiVpc에 IGW CRUD 추가 (/v2.0/internetgateways, external_network_id 사용)
- IGW/RoutingTable/Subnet-attach View 및 URL 신규 (모두 동기)
- API_SPEC.md 3.7.1~3.7.8 네트워크 섹션 추가

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

API_SPEC.md

@@ -387,6 +387,107 @@ DELETE /api/nhn/subnet/{subnet_id}/
 
 ---
 
+### 3.7.1 서브넷에 라우팅 테이블 연결
+
+```
+PUT /api/nhn/subnet/{subnet_id}/attach-routing-table/
+```
+
+**Request Body**
+```json
+{
+  "routingtable_id": "rt-id-123"
+}
+```
+
+---
+
+### 3.7.2 인터넷 게이트웨이 목록 조회
+
+```
+GET /api/nhn/internet-gateway/
+```
+
+**Response (200 OK)** NHN Cloud Internet Gateway Public API `/v2.0/internetgateways` 통과 응답.
+
+---
+
+### 3.7.3 인터넷 게이트웨이 생성
+
+```
+POST /api/nhn/internet-gateway/create/
+```
+
+**Request Body**
+```json
+{
+  "name": "igw-1",
+  "external_network_id": "ext-net-id (선택, 생략 시 자동 조회)"
+}
+```
+
+---
+
+### 3.7.4 인터넷 게이트웨이 상세/삭제
+
+```
+GET    /api/nhn/internet-gateway/{internetgateway_id}/
+DELETE /api/nhn/internet-gateway/{internetgateway_id}/
+```
+
+---
+
+### 3.7.5 라우팅 테이블 목록 조회
+
+```
+GET /api/nhn/routing-table/
+```
+
+---
+
+### 3.7.6 라우팅 테이블 생성
+
+```
+POST /api/nhn/routing-table/create/
+```
+
+**Request Body**
+```json
+{
+  "name": "rt-1",
+  "vpc_id": "vpc-id-123",
+  "distributed": true
+}
+```
+
+---
+
+### 3.7.7 라우팅 테이블 상세/삭제
+
+```
+GET    /api/nhn/routing-table/{routingtable_id}/
+DELETE /api/nhn/routing-table/{routingtable_id}/
+```
+
+---
+
+### 3.7.8 라우팅 테이블 기본 설정 / 게이트웨이 연결·해제
+
+```
+PUT /api/nhn/routing-table/{routingtable_id}/set-default/
+PUT /api/nhn/routing-table/{routingtable_id}/attach-gateway/
+PUT /api/nhn/routing-table/{routingtable_id}/detach-gateway/
+```
+
+**attach-gateway Request Body**
+```json
+{
+  "gateway_id": "igw-id-123"
+}
+```
+
+---
+
 ### 3.8 Floating IP 목록 조회
 
 ```