Add Space metadata model and provisional Forgejo content ADR

Introduce tenant-scoped Space/SpaceMembership with list and detail views.
Document markdown-in-Forgejo as content SoR (ADR-0002). Run migrations on
container start so app.coulomb.social picks up the new tables.

coulomb_social/templates/spaces/detail.html

@@ -0,0 +1,36 @@
+{% extends "base.html" %}
+{% block title %}{{ space.title }} — {{ site_name }}{% endblock %}
+{% block content %}
+  <p class="muted" style="margin:0 0 0.5rem;">
+    <a href="{% url 'core:app_home' %}">← Spaces</a>
+  </p>
+  <h1>{{ space.title }}</h1>
+  {% if space.description %}
+    <p class="muted">{{ space.description }}</p>
+  {% endif %}
+
+  <div class="card">
+    <h2 style="margin-top:0;">Space metadata</h2>
+    <dl>
+      <dt>Slug</dt><dd>{{ space.slug }}</dd>
+      <dt>Tenant</dt><dd>{{ space.tenant_id }}</dd>
+      <dt>Content</dt>
+      <dd>
+        {% if space.has_content_binding %}
+          {{ space.forgejo_owner }}/{{ space.forgejo_repo }}
+          @ {{ space.default_branch }}
+          · root <code>{{ space.content_root }}</code>
+        {% else %}
+          Not bound to a Forgejo repo yet (CSOC-WP-0004-T04).
+        {% endif %}
+      </dd>
+    </dl>
+  </div>
+
+  <div class="card">
+    <h2 style="margin-top:0;">Pages</h2>
+    <p class="muted" style="margin-bottom:0;">
+      Markdown page rendering from Forgejo lands in T04. This view is metadata only.
+    </p>
+  </div>
+{% endblock %}