chore: scaffold chatelet LNbits extension

Nostr-native room rentals (Airbnb-style) for the castle. Establishes the
extension manifest, license, README with the design overview, and static
+ template placeholders. No booking logic yet — subsequent commits build
the model, schema, arbiter, Nostr layer, API, tasks, and wiring bottom-up.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019VUQCfdqiLSsFS2jcGnaFD

templates/chatelet/index.html

@@ -0,0 +1,18 @@
+{% extends "base.html" %} {% from "macros.jinja" import window_vars with context %}
+{% block page %}
+<div class="row q-col-gutter-md">
+  <div class="col-12">
+    <q-card>
+      <q-card-section>
+        <h5 class="text-subtitle1 q-my-none">Chatelet — Room Rentals</h5>
+        <p class="text-caption">
+          Nostr-native room rentals. Operator admin UI goes here (rooms,
+          calendar, bookings). Guest booking UI lives in the webapp.
+        </p>
+      </q-card-section>
+    </q-card>
+  </div>
+</div>
+{% endblock %} {% block scripts %} {{ window_vars(user) }}
+<script src="{{ static_url_for('chatelet/static', 'js/index.js') }}"></script>
+{% endblock %}