mbproxy: add keepalive / connection monitoring

The DL205/DL260 ECOM emits no TCP keepalives, so an idle backend socket
can be silently dropped by a middlebox (switch, firewall, NAT) after
2-5 minutes. Enable OS SO_KEEPALIVE on backend and accepted upstream
sockets, and drive a periodic synthetic FC03 heartbeat on each idle
backend socket so a dead path is detected before a real client request
hits it. Controlled by Connection.Keepalive (ON by default).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mbproxy/src/Mbproxy/Options/ConnectionOptions.cs

@@ -9,4 +9,9 @@ public sealed class ConnectionOptions
     /// graceful shutdown before cancelling them. Default: 10000 (10 s).
     /// </summary>
     public int GracefulShutdownTimeoutMs { get; init; } = 10000;
+
+    /// <summary>
+    /// TCP keepalive and backend-heartbeat connection-monitoring settings. Enabled by default.
+    /// </summary>
+    public KeepaliveOptions Keepalive { get; init; } = new();
 }