feat(auth): forged browser requests are refused (#5031)

#5031 "Forged-request protection: SameSite cookie, per-session CSRF token
on every non-GET, HTMX header". A page on another site could make a
logged-in browser POST to Steward with the session cookie attached.

- steward/auth/csrf.py: a per-session token, checked in one before_request
  for every POST/PUT/PATCH/DELETE (header X-CSRF-Token or form field
  csrf_token, constant-time compare). The bearer-authenticated host-agent
  ingest is exempt; /mcp never reaches Quart.
- Every POST form renders {{ csrf_field() }}; base.html carries the token
  in a meta tag and adds it to every HTMX request; the dashboard's fetch()
  calls send it.
- The session cookie is SameSite=Lax; login issues a fresh token.
- PUBLIC_BASE_URL is now loaded from Settings at startup (it was only set
  when Settings → General was saved).
- tests/test_csrf.py: refusal, acceptance, rotation, the ingest exemption,
  and a guard that every POST form in the templates carries the field.

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

steward/templates/ansible/browse.html

@@ -28,7 +28,7 @@
             {% if view_editable and session.user_role == 'admin' %}
             <a href="/ansible/playbooks/edit/{{ view_source }}/{{ view_path }}" class="btn btn-sm">Edit</a>
             <form method="post" action="/ansible/playbooks/delete" style="display:inline;"
-                  onsubmit="return confirm('Delete {{ view_path }}?');">
+                  onsubmit="return confirm('Delete {{ view_path }}?');">{{ csrf_field() }}
                 <input type="hidden" name="source_name" value="{{ view_source }}">
                 <input type="hidden" name="playbook_path" value="{{ view_path }}">
                 <button type="submit" class="btn btn-sm btn-danger">Delete</button>