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/hosts/form.html

@@ -6,7 +6,7 @@
 <div style="max-width:560px;margin:2rem auto;">
     <h1 class="page-title">{% if host %}Edit Host{% else %}Add Host{% endif %}</h1>
     <div class="card">
-        <form method="post" action="{% if host %}/hosts/{{ host.id }}{% else %}/hosts/{% endif %}">
+        <form method="post" action="{% if host %}/hosts/{{ host.id }}{% else %}/hosts/{% endif %}">{{ csrf_field() }}
             <div class="form-group">
                 <label>Display Name</label>
                 <input type="text" name="name" value="{{ host.name if host else '' }}" required autofocus>