fix(mpc-service): 添加 /api/v1 前缀到 coordinator-client 路径

session-coordinator 的 API 路由注册在 /api/v1/sessions 下，
但 coordinator-client 调用的是 /sessions（404 错误）。

修复所有端点路径:
- /sessions/join -> /api/v1/sessions/join
- /sessions/report-completion -> /api/v1/sessions/report-completion
- /sessions/{id}/status -> /api/v1/sessions/{id}/status
- /sessions/report-failure -> /api/v1/sessions/report-failure

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

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

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

@@ -95,7 +95,7 @@ export class MPCCoordinatorClient implements OnModuleInit {
     this.logger.log(`Joining session: ${request.sessionId}`);
 
     try {
-      const response = await this.client.post('/sessions/join', {
+      const response = await this.client.post('/api/v1/sessions/join', {
         session_id: request.sessionId,
         party_id: request.partyId,
         join_token: request.joinToken,
@@ -127,7 +127,7 @@ export class MPCCoordinatorClient implements OnModuleInit {
     this.logger.log(`Reporting completion for session: ${request.sessionId}`);
 
     try {
-      await this.client.post('/sessions/report-completion', {
+      await this.client.post('/api/v1/sessions/report-completion', {
         session_id: request.sessionId,
         party_id: request.partyId,
         public_key: request.publicKey,
@@ -147,7 +147,7 @@ export class MPCCoordinatorClient implements OnModuleInit {
     this.logger.log(`Getting status for session: ${sessionId}`);
 
     try {
-      const response = await this.client.get(`/sessions/${sessionId}/status`);
+      const response = await this.client.get(`/api/v1/sessions/${sessionId}/status`);
 
       return {
         sessionId: response.data.session_id,
@@ -170,7 +170,7 @@ export class MPCCoordinatorClient implements OnModuleInit {
     this.logger.log(`Reporting failure for session: ${sessionId}`);
 
     try {
-      await this.client.post('/sessions/report-failure', {
+      await this.client.post('/api/v1/sessions/report-failure', {
         session_id: sessionId,
         party_id: partyId,
         error_message: errorMessage,