Add admin-only Settings -> Users page for creating accounts

Peter needed a second user for throwaway testing without going through
Django admin each time. Superusers now get a "Users" tab in Settings
(hidden for regular users) with a create-user form and a list of existing
accounts. New users are regular (non-staff, non-superuser) by default, with
an optional "Grant admin access" checkbox; creating one immediately issues
their API token and displays it once, since that's the credential an agent
account needs and a human account can otherwise regenerate for themselves
later from their own Settings -> Account.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/artifacts/settings_users.html

@@ -0,0 +1,31 @@
+{% extends "artifacts/settings_base.html" %}
+
+{% block settings_content %}
+  <h2>Users</h2>
+
+  {% if created_token %}
+    <p><strong>API token for {{ created_user.username }}</strong> — shown once, copy it now:</p>
+    <p><code class="token-value">{{ created_token.key }}</code></p>
+  {% endif %}
+
+  <h3>Add User</h3>
+  <p>New users are regular accounts by default — they can log in and use their own token, but cannot reach Django admin unless you grant admin access below.</p>
+  <form method="post">
+    {% csrf_token %}
+    <p><label>Username<br><input type="text" name="username" required autocomplete="off"></label></p>
+    <p><label>Password<br><input type="password" name="password" required autocomplete="new-password"></label></p>
+    <p><label><input type="checkbox" name="is_superuser"> Grant admin access</label></p>
+    <button type="submit">Create User</button>
+  </form>
+
+  <h3>Existing Users</h3>
+  <ul class="storage-facts">
+    {% for user in users %}
+      <li>
+        {{ user.username }}{% if user == request.user %} (you){% endif %}
+        {% if user.is_superuser %} &mdash; admin{% endif %}
+        {% if not user.is_active %} &mdash; disabled{% endif %}
+      </li>
+    {% endfor %}
+  </ul>
+{% endblock %}