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/schedules.html

@@ -54,15 +54,15 @@
       {% else %}due now{% endif %}
     </td>
     <td class="td-actions" style="white-space:nowrap;">
-      <form method="post" action="/ansible/schedules/{{ s.id }}/run-now" style="display:inline;">
+      <form method="post" action="/ansible/schedules/{{ s.id }}/run-now" style="display:inline;">{{ csrf_field() }}
         <button class="btn btn-sm btn-ghost" type="submit">Run now</button>
       </form>
-      <form method="post" action="/ansible/schedules/{{ s.id }}/toggle" style="display:inline;">
+      <form method="post" action="/ansible/schedules/{{ s.id }}/toggle" style="display:inline;">{{ csrf_field() }}
         <button class="btn btn-sm btn-ghost" type="submit">{{ "Pause" if s.enabled else "Resume" }}</button>
       </form>
       <a href="?edit={{ s.id }}#form" class="btn btn-sm btn-ghost">Edit</a>
       <form method="post" action="/ansible/schedules/{{ s.id }}/delete" style="display:inline;"
-            onsubmit="return confirm('Delete schedule {{ s.name }}?');">
+            onsubmit="return confirm('Delete schedule {{ s.name }}?');">{{ csrf_field() }}
         <button class="btn btn-sm btn-danger" type="submit">Delete</button>
       </form>
     </td>
@@ -79,7 +79,7 @@
 {% set declared_names = (edit_variables or []) | map(attribute='name') | list %}
 <div class="card" id="form">
   <h2 class="section-title" style="margin-bottom:1rem;">{{ "Edit schedule" if editing else "New schedule" }}</h2>
-  <form method="post" action="{{ '/ansible/schedules/' ~ editing.id if editing else '/ansible/schedules' }}">
+  <form method="post" action="{{ '/ansible/schedules/' ~ editing.id if editing else '/ansible/schedules' }}">{{ csrf_field() }}
     <div style="display:grid;grid-template-columns:repeat(auto-fit,minmax(220px,1fr));gap:1rem;">
       <div class="form-group" style="margin-bottom:0;">
         <label>Name</label>