feat: add Python packaging metadata and a Nix flake (#16)

## What

- `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 that console script; `main` is a coroutine and cannot be referenced by one. The `__main__` path behaves as before.
- `flake.nix` / `package.nix` — the package, `overlays.default`, a dev shell, and `checks`. The pytest suite runs as part of the build.
- `module.nix` — a NixOS module: `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`, takes `CAP_NET_BIND_SERVICE` only while a listener is on a privileged port, and opens no firewall ports.
- README: pip install, Nix usage, and a NixOS service example.

## Why

The proxy could only be consumed as a container. Anyone deploying it on NixOS had to vendor a package definition into their own configuration — which is exactly what happened downstream, and is now deleted there in favour of this.

## Not in scope

- The `Dockerfile` and its CI workflow are untouched.
- No Nix job in CI; the runner has no Nix. The build is reproducible locally with `nix flake check`.
- `version` is static and tracks the latest tag (`0.3.0`); bump it with the tag.

## Verification

- `nix flake check` — package builds, 14 tests pass inside the build.
- Consumed from a downstream NixOS host with `--override-input`: the unit's `ExecStart` resolves to the module's own build, and that flake's checks pass too.

---------

Co-authored-by: Emma Thorpe <emma.thorpe@citrix.com>
Reviewed-on: #16

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
+}