fix(identity-service): 修复 MPC 服务调用路径

问题: identity-service 调用 mpc-service 返回 404
原因: mpc-service 有全局前缀 /api/v1，但调用路径缺少此前缀

修复:
- mpc-client.service.ts: 添加 /api/v1 前缀到 MPC API 调用路径
- kong.yml: 添加 /api/v1/mpc-party 路由

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/api-gateway/kong.yml

@@ -115,6 +115,10 @@ services:
         paths:
           - /api/v1/mpc
         strip_path: false
+      - name: mpc-party-api
+        paths:
+          - /api/v1/mpc-party
+        strip_path: false
 
   # ---------------------------------------------------------------------------
   # Leaderboard Service - 排行榜服务

backend/services/identity-service/src/infrastructure/external/mpc/mpc-client.service.ts

@@ -107,7 +107,7 @@ export class MpcClientService {
       // 调用 mpc-service 的 keygen 同步接口
       const response = await firstValueFrom(
         this.httpService.post<KeygenResult>(
-          `${this.mpcServiceUrl}/mpc-party/keygen/participate-sync`,
+          `${this.mpcServiceUrl}/api/v1/mpc-party/keygen/participate-sync`,
           {
             sessionId: request.sessionId,
             partyId: 'server-party',
@@ -157,7 +157,7 @@ export class MpcClientService {
       // 调用 mpc-service 的 signing 同步接口
       const response = await firstValueFrom(
         this.httpService.post<SigningResult>(
-          `${this.mpcServiceUrl}/mpc-party/signing/participate-sync`,
+          `${this.mpcServiceUrl}/api/v1/mpc-party/signing/participate-sync`,
           {
             sessionId: request.sessionId,
             partyId: 'server-party',