Listen on the socket systemd hands over, so a restart never answers 502 (#218)

ddiff --git a/apps/server/src/main.ts b/apps/server/src/main.ts
iindex 0ac6e31..cda64ac 100644
---- a/apps/server/src/main.ts
++++ b/apps/server/src/main.ts
@@@ -1382,9 +1382,14 @@ async function main(): Promise<void> {
        socket.destroy()
      }
    })
--  http.listen(config.port, config.host, () => {
--    log.info('listening', { host: config.host, port: config.port })
--  })
++  // Socket activation (2026-10-31): systemd's .socket unit holds the listener across restarts, so a deploy
++  // queues requests in the kernel instead of answering 502. Without the socket unit, the usual listen.
++  const socketActivated = process.env.LISTEN_FDS === '1' && Number(process.env.LISTEN_PID) === process.pid
++  const onListening = (): void => {
++    log.info('listening', socketActivated ? { fd: 3 } : { host: config.host, port: config.port })
++  }
++  if (socketActivated) http.listen({ fd: 3 }, onListening)
++  else http.listen(config.port, config.host, onListening)
  
    // Scheduler loop with drift correction — never tied to timers' jitter. It
    // runs at the fastest room rate; each room's accumulator in InstanceHub