refactor(flake): user registry, multi-user hosts, and portable home outputs (#49)

## Summary

Separates user identity (data) from the reusable Nix modules and lets a host declare any number of users, replacing the previous one-user-per-host structure. Also restructures the tree and exposes the home config for use off these hosts.

## Changes

- **User registry** (`users/registry.nix`): per-user identity (name, email, groups, authorized + signing keys) as the single source of truth; no user data hardcoded in modules.
- **Multi-user `mkHost`**: a host declares a `users` set keyed by username; per-user identity is injected into each home config via the `identity` module arg.
- **Restructured layout**: `users/`, `home/`, `modules/`, `hosts/`, `lib/` replace the former `lyrathorpe/` and `system/` trees.
- **Portable outputs**: standalone `homeConfigurations."<user>@<system>"` (the portable subset — shell, git, editor, claude) plus an exported `homeModules` for use on machines not managed by this flake, or as an input to other flakes.
- Docs (`README.md`, `home/README.md`) and `.gitignore` updated for the new paths.

## Fixes

- Closes #46 — shared user module authorized one user's SSH key for every account.
- Closes #47 — git committer identity hardcoded as defaults instead of per-user.
- Closes #48 — EDaaS systemd linger hardcoded to a literal username.

## Verification

- `nix flake check` passes: treefmt, deadnix, statix, pre-commit, and evaluation of all NixOS hosts + Darwin + homeConfigurations.
- Derivation-path comparison vs `main`: `lyrathorpe-mbp` and `emmathorpe-edaas` are byte-identical; `lyrathorpe-t400`, `lyrathorpe-macpro31` and `lyrathorpe-rpi5` differ only by de-duplicating a repeated `authorized_keys` entry (confirmed with nix-diff — no other change).
- Standalone `homeConfigurations."lyrathorpe@x86_64-linux".activationPackage` builds.

## Notes

- `emmathorpe` has no personal authorized key yet (it previously inherited Lyra's key via the bug in #46); the registry entry is intentionally empty — add a real key if SSH login as `emmathorpe` is wanted (moot on the WSL host).
- A two-repo (public dotfiles / private systems) split is deferred by design; this internal restructure is the prerequisite for it.

---------

Co-authored-by: Emma Thorpe <emma.thorpe@citrix.com>
Reviewed-on: #49

hosts/RPi5/reverse-proxy.nix

@@ -0,0 +1,39 @@
+# Native nginx reverse proxy. The proxy configuration is declarative Nix:
+# every proxied service is an entry under services.nginx.virtualHosts, so the
+# whole routing table lives in this file and is built/version-controlled with
+# the rest of the system.
+#
+# To add a proxied service, add another virtualHosts."<host>" entry following
+# the example below. To serve it over HTTPS, uncomment enableACME + forceSSL on
+# that vhost once it has a real DNS name and the ACME HTTP-01/DNS-01 challenge
+# can be satisfied (see security.acme for the account/email and DNS settings).
+{ ... }:
+{
+  services.nginx = {
+    enable = true;
+    recommendedProxySettings = true; # sane proxy_set_header defaults (Host, X-Forwarded-*)
+    recommendedTlsSettings = true;
+    recommendedOptimisation = true;
+    recommendedGzipSettings = true;
+
+    virtualHosts = {
+      # Example reverse-proxy vhost. Replace the name and upstream with a real
+      # service (e.g. a container published by the Docker host on this machine).
+      "example.lan" = {
+        # enableACME = true;   # request a Let's Encrypt cert for this host
+        # forceSSL = true;     # redirect HTTP -> HTTPS once the cert exists
+        locations."/" = {
+          proxyPass = "http://127.0.0.1:8080";
+          proxyWebsockets = true; # forward Upgrade/Connection for WebSocket apps
+        };
+      };
+    };
+  };
+
+  # Public reverse-proxy ports. 443 is opened now so flipping a vhost to TLS
+  # needs no firewall change.
+  networking.firewall.allowedTCPPorts = [
+    80
+    443
+  ];
+}