fix(gateway): support localhost dev ports in CORS

[stablegenius49]

Summary

• enable FastAPI CORSMiddleware in the gateway for direct backend access in local dev
• add a localhost/127.0.0.1 regex fallback so non-3000 frontend ports (e.g. 3001/5173) are accepted
• trim and normalize CORS_ORIGINS parsing and add regression tests for config + middleware wiring

Verification

• cd backend && python3 -m py_compile src/gateway/config.py src/gateway/app.py tests/test_gateway_cors.py
• cd backend && pytest -q tests/test_gateway_cors.py (fails in this environment: ModuleNotFoundError: fastapi; backend deps are not installed here)

Closes #47

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[WillemJiang]

@stablegenius49 Thanks for your contribution. Please click the CLA button to sign the CLA first, and then we can review your PR.