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/inventory/groups.html

@@ -12,7 +12,7 @@
 
 <div class="card" style="margin-bottom:1.5rem;">
     <h3 class="section-title">New Group</h3>
-    <form method="post" action="/ansible/inventory/groups" style="display:flex;gap:0.75rem;align-items:flex-end;">
+    <form method="post" action="/ansible/inventory/groups" style="display:flex;gap:0.75rem;align-items:flex-end;">{{ csrf_field() }}
         <div class="form-group" style="flex:1;margin:0;">
             <label>Name</label>
             <input type="text" name="name" placeholder="webservers" required>
@@ -38,7 +38,7 @@
         <td style="color:var(--text-muted);font-size:0.9rem;">{{ grp.targets | length }}</td>
         <td class="td-actions">
             <a href="/ansible/inventory/groups/{{ grp.id }}" class="btn btn-sm btn-ghost">Edit</a>
-            <form method="post" action="/ansible/inventory/groups/{{ grp.id }}/delete" style="display:inline;">
+            <form method="post" action="/ansible/inventory/groups/{{ grp.id }}/delete" style="display:inline;">{{ csrf_field() }}
                 <button type="submit" class="btn btn-sm btn-danger"
                         onclick="return confirm('Delete group {{ grp.name }}?')">×</button>
             </form>