[RajGahoi]

Closes #153

Summary

1. Adds \SubjectKeyIdentifier\ to the ephemeral test CA certificate in \Backend/tests/test_sso.py.
2. Adds \SubjectKeyIdentifier\ and \AuthorityKeyIdentifier\ (referencing the test CA) to the generated server certificate in \make_server_cert(). This satisfies Python 3.13's default \ssl.VERIFY_X509_STRICT\ requirement without turning off strict certificate verification.
3. Adds '3.13'\ to the backend CI matrix (.github/workflows/ci.yml), keeping flake8 lint restricted to target Python 3.12.

Testing

• Validated certificate generation and extension bindings locally.
• Python syntax compilation verified on \Backend/tests/test_sso.py.

[RajGahoi]

All 26/26 CI checks including the Python 3.10, 3.12, and 3.13 backend matrix and security invariants have passed green. Ready for review when convenient! Thank you.