feat: add Python packaging metadata and a Nix flake

Package the proxy properly so it can be consumed outside a container, and
without downstream users vendoring a package definition into their own
configuration.

- pyproject.toml: setuptools metadata with a `legacy-email-proxy` console
  script. Runtime dependencies are read dynamically from requirements.txt, so
  the Docker build and the package cannot drift apart.
- proxy_server.run(): a synchronous entry point for the console script, since
  `main` is a coroutine and cannot be referenced by one directly. The
  `__main__` path is unchanged in behaviour.
- package.nix / flake.nix: the package, an overlay, and a dev shell. The test
  suite runs as part of the build, so `nix flake check` covers it.
- module.nix: a NixOS module exposing `services.legacy-email-proxy` with
  freeform `settings` (environment variables) and a separate `environmentFile`
  for credentials, so secrets stay out of the Nix store. Runs under
  DynamicUser with CAP_NET_BIND_SERVICE only while a listener needs a
  privileged port, and opens no firewall ports.

The Dockerfile and its CI workflow are deliberately untouched.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

proxy_server.py

@@ -458,8 +458,17 @@ async def main():
         await pop3_server.serve_forever()
 
 
-if __name__ == "__main__":
+def run():
+    """Run the proxy until interrupted.
+
+    Entry point for the ``legacy-email-proxy`` console script; ``main`` is a
+    coroutine and cannot be referenced directly by one.
+    """
     try:
         asyncio.run(main())
     except KeyboardInterrupt:
         logger.info("Shutdown requested")
+
+
+if __name__ == "__main__":
+    run()