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

python/gems/templates/room.html

@@ -0,0 +1,17 @@
+{% extends "base.html" %}
+{% block title %}Room {{ room.code }} · Gems{% endblock %}
+{% block main_class %}board-main{% endblock %}
+{% block topbar %}
+<header class="topbar room-topbar">
+  <a href="/" class="brand"><span class="brand-mark">◆</span> Gems</a>
+  <div class="room-identity"><span class="eyebrow">Room</span><strong>{{ room.code }}</strong></div>
+  <label class="invite-link room-invite">Invite link<input readonly value="{{ public_origin }}/join/{{ room.code }}" aria-label="Invite link"></label>
+</header>
+{% endblock %}
+{% block content %}
+<div class="room-shell">
+  <div id="room-state" hx-ext="sse" sse-connect="/rooms/{{ room.code }}/events" sse-swap="room">
+    {% include "partials/room_state.html" %}
+  </div>
+</div>
+{% endblock %}