feat(timeline): record a change by hand from the Timeline, a host's page or MCP log_change (#6350)

A change Steward can't see happen (a config edited, a disk swapped) is
recorded by whoever made it: what was done, its area, when (in the reader's
own zone), the hosts, what it was done to and a detail. Its author or an
admin edits or deletes it; recording, editing and deleting are audited and
those audit rows stay off the timeline. log_change records one as
"<user> via MCP", editable by the token's user.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/templates/timeline/list.html

@@ -8,6 +8,7 @@
 {% block title %}Timeline — Steward{% endblock %}
 {% block content %}
 {% call page_header("Timeline", "What happened, newest first: alerts, playbook runs and changes, on every host.") %}
+  {% if session.user_role in ('operator', 'admin') %}<a class="btn btn-sm" href="/timeline/changes/new?{{ ({'host': filters.host, 'back': back} if filters.host else {'back': back})|urlencode }}">Record a change</a>{% endif %}
   {% if session.user_role == 'admin' %}<a class="btn btn-ghost btn-sm" href="/settings/timeline/">Areas and types</a>{% endif %}
 {% endcall %}