fix(mpc-service): 修复数据库provider从mysql改为postgresql

mpc-service错误配置为mysql，与项目统一使用的postgresql不一致，
导致服务启动时Prisma验证DATABASE_URL失败。

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

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

backend/services/mpc-service/prisma/schema.prisma

@@ -7,7 +7,7 @@ generator client {
 }
 
 datasource db {
-  provider = "mysql"
+  provider = "postgresql"
   url      = env("DATABASE_URL")
 }