fix: add pythonpath to pytest config for CI compatibility

Without pythonpath=["src"], pytest can't resolve `from module.xxx`
imports when running from the backend/ directory in CI.

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

Author: EstrellaXD

backend/pyproject.toml

@@ -37,6 +37,7 @@ dev = [
 
 [tool.pytest.ini_options]
 testpaths = ["src/test"]
+pythonpath = ["src"]
 asyncio_mode = "auto"
 
 [tool.ruff]