Add lightweight checksession / login check

[leastprivilege]

for same site frontends

[brockallen]

silent login

• new ~/bff/silentlogin endpoint to trigger challenge
• hook OIDC events for redirect for login (pass prompt=none) and error (handle error=login_required)
• callback page with JS to postMessage to parent with success/failure
• sample JS to create iframe and handle onmessage

[github-actions]

This issue has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue.