Relay: end a user's sessions into a vault the moment they lose access to it

src/DodoSSH.Domain/Relay.cs

@@ -120,4 +120,7 @@ public enum RelayCloseReason
 
     /// <summary>An error ended the session.</summary>
     Error = 7,
+
+    /// <summary>The user lost access to the host's vault while the session was open.</summary>
+    AccessRevoked = 8,
 }