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>

plugins/host_agent/templates/panel.html

@@ -20,17 +20,17 @@
     <a href="/plugins/host_agent/{{ host.id }}/" class="btn btn-sm btn-ghost">Full metrics →</a>
     {% if session.user_role == 'admin' %}
     {% if ansible_available and target %}
-    <form method="post" action="/plugins/host_agent/update" style="margin:0;">
+    <form method="post" action="/plugins/host_agent/update" style="margin:0;">{{ csrf_field() }}
       <input type="hidden" name="inventory_scope" value="{{ scope }}">
       <button type="submit" class="btn btn-sm" title="Refresh agent.py + restart (token preserved)">Update agent</button>
     </form>
     {% endif %}
     <form method="post" action="/plugins/host_agent/fleet/{{ host.id }}/rotate-token" style="margin:0;"
-          onsubmit="return confirm('Rotate token? The agent stops reporting until reinstalled/updated with the new token.');">
+          onsubmit="return confirm('Rotate token? The agent stops reporting until reinstalled/updated with the new token.');">{{ csrf_field() }}
       <button type="submit" class="btn btn-sm btn-ghost">Rotate token</button>
     </form>
     <form method="post" action="/plugins/host_agent/fleet/{{ host.id }}/delete" style="margin:0;"
-          onsubmit="return confirm('Remove this agent registration? Metrics stop until re-registered.');">
+          onsubmit="return confirm('Remove this agent registration? Metrics stop until re-registered.');">{{ csrf_field() }}
       <button type="submit" class="btn btn-sm btn-danger">Remove</button>
     </form>
     {% endif %}
@@ -49,7 +49,7 @@
       Connects over a one-time bootstrap user + password (not stored).
     </p>
     <form method="post" action="/plugins/host_agent/provision"
-          style="display:flex;gap:0.6rem;align-items:flex-end;flex-wrap:wrap;">
+          style="display:flex;gap:0.6rem;align-items:flex-end;flex-wrap:wrap;">{{ csrf_field() }}
       <input type="hidden" name="inventory_scope" value="{{ scope }}">
       <div class="form-group" style="margin:0;">
         <label>Bootstrap user</label>
@@ -96,7 +96,7 @@
               border-radius:6px;padding:0.7rem 0.9rem;display:flex;align-items:center;gap:0.8rem;flex-wrap:wrap;">
     <span style="color:var(--yellow);">⚠</span>
     <span style="font-size:0.84rem;flex:1;min-width:13rem;">No managed SSH key yet — Steward needs one to log into hosts.</span>
-    <form method="post" action="/settings/ansible/generate-key" style="margin:0;">
+    <form method="post" action="/settings/ansible/generate-key" style="margin:0;">{{ csrf_field() }}
       <input type="hidden" name="next" value="/hosts/{{ host.id }}">
       <button type="submit" class="btn btn-sm">Generate managed key</button>
     </form>
@@ -115,7 +115,7 @@
   </p>
   {# Provision: brand-new host (creates steward account + key over a one-time password) #}
   <form method="post" action="/plugins/host_agent/provision"
-        style="display:flex;gap:0.6rem;align-items:flex-end;flex-wrap:wrap;margin-bottom:0.75rem;">
+        style="display:flex;gap:0.6rem;align-items:flex-end;flex-wrap:wrap;margin-bottom:0.75rem;">{{ csrf_field() }}
     <input type="hidden" name="inventory_scope" value="{{ scope }}">
     <div style="font-size:0.78rem;color:var(--text-muted);width:100%;">Provision (first contact — fresh host):</div>
     <div class="form-group" style="margin:0;">
@@ -129,7 +129,7 @@
     <button type="submit" class="btn btn-sm">Provision</button>
   </form>
   {# Install: host already has the steward account (managed key works) #}
-  <form method="post" action="/plugins/host_agent/deploy" style="display:flex;gap:0.6rem;align-items:center;">
+  <form method="post" action="/plugins/host_agent/deploy" style="display:flex;gap:0.6rem;align-items:center;">{{ csrf_field() }}
     <input type="hidden" name="inventory_scope" value="{{ scope }}">
     <span style="font-size:0.78rem;color:var(--text-muted);">Already provisioned?</span>
     <button type="submit" class="btn btn-sm btn-ghost">Install agent (managed key)</button>
@@ -138,7 +138,7 @@
 
   {% if reg and session.user_role == 'admin' %}
   <form method="post" action="/plugins/host_agent/fleet/{{ host.id }}/delete" style="margin:0.75rem 0 0;"
-        onsubmit="return confirm('Clear this pending registration? Its token is discarded.');">
+        onsubmit="return confirm('Clear this pending registration? Its token is discarded.');">{{ csrf_field() }}
     <button type="submit" class="btn btn-sm btn-ghost">Clear pending registration</button>
   </form>
   {% endif %}