feat(gems): add multiplayer gem game with custom content packs

- implement FastAPI and HTMX lobby and game interfaces
- support up to four human and AI-controlled players
- add configurable rules, victory conditions, and expansion modules
- support validated JSON cards, patrons, objectives, and outposts

systems/jeeves/services/gems.nix

@@ -0,0 +1,52 @@
+{
+  inputs,
+  pkgs,
+  ...
+}:
+let
+  vars = import ../vars.nix;
+  stateDir = "${vars.services}/gems";
+in
+{
+  users.groups.gems = { };
+  users.users.gems = {
+    isSystemUser = true;
+    group = "gems";
+    home = stateDir;
+  };
+
+  systemd.tmpfiles.rules = [
+    "d ${stateDir} 0750 gems gems - -"
+  ];
+
+  systemd.services.gems = {
+    description = "Gems multiplayer game";
+    after = [ "network.target" ];
+    wantedBy = [ "multi-user.target" ];
+
+    environment = {
+      PYTHONPATH = "${inputs.self}";
+      GEMS_DATABASE_PATH = "${stateDir}/gems.sqlite3";
+      GEMS_KEY_PATH = "${stateDir}/instance.key";
+      GEMS_PUBLIC_ORIGIN = "https://gems.tmmworkshop.com";
+      GEMS_SECURE_COOKIES = "true";
+    };
+
+    serviceConfig = {
+      Type = "simple";
+      User = "gems";
+      Group = "gems";
+      ExecStart = "${pkgs.my_python}/bin/python -m python.gems.main --host 127.0.0.1 --port 8002";
+      Restart = "on-failure";
+      RestartSec = "5s";
+      StandardOutput = "journal";
+      StandardError = "journal";
+      NoNewPrivileges = true;
+      PrivateTmp = true;
+      ProtectHome = true;
+      ProtectSystem = "strict";
+      ReadOnlyPaths = [ "${inputs.self}" ];
+      ReadWritePaths = [ stateDir ];
+    };
+  };
+}