fix(mpc-service): 使用标准 migrate deploy 替代 db push

移除 db push --accept-data-loss，改用 prisma migrate deploy
这是创建账号流程涉及的第3个服务（identity, backup, mpc）

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

backend/services/mpc-service/docker-entrypoint.sh

@@ -2,12 +2,12 @@
 set -e
 
 echo "=== MPC Service Starting ==="
-echo "Syncing database schema..."
+echo "Running database migrations..."
 
-# Sync database schema (create tables if not exist)
-npx prisma db push --accept-data-loss
+# Run migrations (standard production approach - fail if migration fails)
+npx prisma migrate deploy
 
-echo "Database schema synced"
+echo "Database migrations completed"
 echo "Starting application..."
 
 # Start the application