Send a goodbye message to clients on signal-initiated shutdown

SIGTERM and SIGINT canceled the server context directly, so clients
were force-closed without the TranDisconnectMsg broadcast that the API
shutdown endpoint sends. The signal handler now calls Server.Shutdown
with a goodbye message, giving both shutdown paths the same behavior:
broadcast, a brief flush window, then force-close and drain.

Default signal handling is restored before the graceful shutdown
begins, so a second signal still kills the process immediately rather
than waiting out the flush window.

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