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/playbook_editor.html

@@ -15,7 +15,7 @@
   automatically; you can also configure a local-dir source under <a href="/settings/ansible/">Settings → Ansible</a>.
 </div>
 {% else %}
-<form method="post" action="/ansible/playbooks/save">
+<form method="post" action="/ansible/playbooks/save">{{ csrf_field() }}
   <input type="hidden" name="editing" value="{{ '1' if editing else '0' }}">
   <div style="display:flex;gap:1rem;flex-wrap:wrap;margin-bottom:1rem;">
     <div class="form-group" style="margin-bottom:0;">
@@ -54,7 +54,7 @@
 
 {% if editing %}
 <form method="post" action="/ansible/playbooks/delete" style="margin-top:1rem;"
-      onsubmit="return confirm('Delete {{ playbook_path }}?');">
+      onsubmit="return confirm('Delete {{ playbook_path }}?');">{{ csrf_field() }}
   <input type="hidden" name="source_name" value="{{ source_name }}">
   <input type="hidden" name="playbook_path" value="{{ playbook_path }}">
   <button type="submit" class="btn btn-danger btn-sm">Delete playbook</button>