fix: replace python-jose with PyJWT for robust JWKS signature verification

python-jose failed to correctly construct RSA public keys from Microsoft
JWKS entries lacking an explicit alg field, causing signature verification
failures. Switch auth.py to PyJWT + jwt.algorithms.RSAAlgorithm.from_jwk()
which handles Entra JWKS correctly. Add cryptography explicitly to deps.
Update auth tests to remove unused python-jose fixture code.

backend/requirements.txt

@@ -4,7 +4,9 @@ pymongo
 python-dotenv
 requests
 PyYAML
+PyJWT
 python-jose[cryptography]
+cryptography
 pydantic-settings
 structlog
 tenacity