add SSO session extend flow (management)

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.

management/server/peer/peer.go

@@ -367,6 +367,22 @@ func (p *Peer) LoginExpired(expiresIn time.Duration) (bool, time.Duration) {
 	return timeLeft <= 0, timeLeft
 }
 
+// SessionExpiresAt returns the absolute UTC instant at which the peer's SSO
+// session expires, derived from LastLogin and the account-level
+// PeerLoginExpiration setting. Returns the zero value when login expiration
+// does not apply (peer not SSO-registered, peer-level toggle off, or account
+// expiry disabled). Callers should treat the zero value as "no deadline".
+func (p *Peer) SessionExpiresAt(accountExpirationEnabled bool, expiresIn time.Duration) time.Time {
+	if !accountExpirationEnabled || !p.AddedWithSSOLogin() || !p.LoginExpirationEnabled {
+		return time.Time{}
+	}
+	last := p.GetLastLogin()
+	if last.IsZero() {
+		return time.Time{}
+	}
+	return last.Add(expiresIn).UTC()
+}
+
 // FQDN returns peers FQDN combined of the peer's DNS label and the system's DNS domain
 func (p *Peer) FQDN(dnsDomain string) string {
 	if dnsDomain == "" {