Adds the management-server half of the SSO session-extension feature:
- New ExtendAuthSession gRPC RPC that refreshes a peer's session expiry
using a fresh JWT, validated through the same pipeline as Login but
without tearing down the tunnel or redoing the NetworkMap sync.
- Per-peer SessionExpiresAt timestamp on every LoginResponse and
SyncResponse so connected clients learn the deadline on the existing
long-lived stream, and admin-side changes (toggling expiration,
changing the expiration window) reach every peer within seconds.
- SessionExpiresAt(...) helper on Peer that derives the absolute UTC
deadline from LastLogin + the account-level PeerLoginExpiration
setting, returning zero when the peer is not SSO-tracked or expiration
is disabled.
The matching client-side consumer of these fields lands separately.
6dcba89a46