fix(blockchain): 修复 MPC 签名 API 路径

mpc-service 使用全局前缀 api/v1，调整签名 API 路径：
- /mpc/sign → /api/v1/mpc/sign
- /mpc/sign/:sessionId/status → /api/v1/mpc/sign/:sessionId/status

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/blockchain-service/src/infrastructure/mpc/mpc-signing.client.ts

@@ -86,7 +86,7 @@ export class MpcSigningClient {
         sessionId: string;
         status: string;
       }>(
-        `${this.mpcServiceUrl}/mpc/sign`,
+        `${this.mpcServiceUrl}/api/v1/mpc/sign`,
         {
           username: this.hotWalletUsername,
           messageHash,
@@ -115,7 +115,7 @@ export class MpcSigningClient {
         status: string;
         signature?: string;
       }>(
-        `${this.mpcServiceUrl}/mpc/sign/${sessionId}/status`,
+        `${this.mpcServiceUrl}/api/v1/mpc/sign/${sessionId}/status`,
         {
           headers: { 'Content-Type': 'application/json' },
           timeout: 10000,