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/_install_result.html

@@ -10,7 +10,7 @@
         hx-post="/settings/plugins/restart/"
         hx-target="#restart-banner"
         hx-swap="outerHTML"
-        style="display:inline;margin:0;">
+        style="display:inline;margin:0;">{{ csrf_field() }}
     <button type="submit"
             style="background:none;border:none;padding:0;color:var(--yellow);cursor:pointer;font-size:0.82rem;text-decoration:underline;">
       Restart now