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

overlays/default.nix

@@ -24,6 +24,7 @@
         fastapi
         fastapi-cli
         httpx
+        jinja2
         mypy
         pgvector
         psycopg
@@ -34,11 +35,13 @@
         pytest-mock
         pytest-xdist
         python-multipart
+        pydantic-settings
         ruff
         sqlalchemy
         tenacity
         tinytuya
         typer
+        uvicorn
         websockets
       ]
     );