Add pytest-xdist to requirements for parallel test execution

backend/requirements.txt

@@ -37,6 +37,7 @@ apscheduler==3.11.0
 pytest>=8.0.0
 pytest-asyncio>=0.23.5
 pytest-cov>=4.1.0
+pytest-xdist>=3.8.0
 requests>=2.32.0
 
 # Development tools