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>

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1787135253,
+        "narHash": "sha256-RD2kNWCG+Bjo6h+JVjWVNntZs2GtRoeY2xHjts/FNkA=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "ffb3c9b700e759be2ef13237c9d8f953b32a1e46",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}