support systemd socket activation

[syphar]

nginx currently returns 502s during deployments and webserver restarts. Across the last three restart windows, we observed 267–689 connection-refused errors per restart. These occur between the old webserver closing its listener during graceful shutdown and the new process accepting connections.

This PR adds systemd socket activation support to both docs_rs_web and the legacy daemon:

• systemd opens the TCP listening socket and keeps it open across service restarts.
• The webserver adopts the inherited socket instead of binding the port itself.
• New connections queue in the socket backlog while the webserver restarts; existing requests use the current graceful shutdown path.

Without an inherited socket, the webserver binds its configured address as before. Socket activation should prevent these brief connection-refused outages, provided the backlog has capacity and requests stay within proxy timeouts.

I'll work on the server config once this change is deployed.

more info:

• systemd for Developers I: Socket Activation - Introduction to socket activation,
  inherited listening sockets, and application support.
• systemd.socket reference - Configuration details for ListenStream, Accept=no, Backlog, and socket/service units.
• systemd-socket-activate reference - Testing socket activation locally.
• sd_listen_fds reference - The descriptor-passing protocol handled by listenfd, including LISTEN_PID, LISTEN_FDS, and FD 3.

[syphar]

r? @GuillaumeGomez

[GuillaumeGomez]

Thanks!

[syphar]

ah damn.... I forgot to rebase / rename commits here on this PR, now it's on main 😅

[syphar]

I rebased / force-pushed on main to fix the commit