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/dashboard/_widget_drawer.html

@@ -27,7 +27,7 @@
         </summary>
         <form action="/d/{{ dashboard.id }}/edit/add/{{ w.key }}" method="post"
               class="add-widget-form"
-              style="padding:0.75rem 0.85rem;padding-top:0;border-top:1px solid var(--border);">
+              style="padding:0.75rem 0.85rem;padding-top:0;border-top:1px solid var(--border);">{{ csrf_field() }}
           <div style="display:grid;gap:0.5rem;padding-top:0.75rem;">
             <div>
               <label style="font-size:0.74rem;color:var(--text-muted);display:block;margin-bottom:0.2rem;">