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/auth/setup.html

@@ -5,7 +5,7 @@
     <div class="card">
         <h2 class="page-title" style="margin-bottom:0.5rem;">First-Run Setup</h2>
         <p style="margin-bottom:1.5rem;color:var(--text-muted);font-size:0.9rem;">Create the initial admin account.</p>
-        <form method="post" action="/setup">
+        <form method="post" action="/setup">{{ csrf_field() }}
             <div class="form-group">
                 <label>Username</label>
                 <input type="text" name="username" autofocus required>