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/settings/reports.html

@@ -20,7 +20,7 @@
 <div style="max-width:560px;">
 
   {# ── Schedule ─────────────────────────────────────────────────────────────── #}
-  <form method="post" action="/settings/reports/">
+  <form method="post" action="/settings/reports/">{{ csrf_field() }}
   <div class="card" style="margin-bottom:1.25rem;">
     <h2 class="section-title" style="margin-bottom:1.25rem;">Weekly Digest</h2>
 
@@ -84,7 +84,7 @@
     <p style="font-size:0.85rem;color:var(--text-muted);margin-bottom:1rem;">
       Send a report immediately regardless of schedule. Useful for testing.
     </p>
-    <form method="post" action="/settings/reports/send-now/">
+    <form method="post" action="/settings/reports/send-now/">{{ csrf_field() }}
       <button type="submit" class="btn btn-ghost">Send report now</button>
     </form>
   </div>