Add cryptography dependency for MySQL 8 auth

pymysql needs the cryptography package to speak MySQL 8's default
caching_sha2_password, so 'flask db upgrade' against a stock MySQL 8
failed with 'cryptography package is required'. Make it a real
dependency (dev, prod, CI all connect cleanly) and drop the CI
native-auth workaround that stood in for it.

requirements.txt

@@ -83,6 +83,7 @@ pygments==2.20.0
 pyjwt==2.12.1
     # via flask-jwt-extended
 pymysql==1.1.3
+cryptography>=42.0
     # via -r requirements.in
 pytest==9.0.3
     # via