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>

plugins/docker/templates/docker/disk.html

@@ -54,19 +54,19 @@
     <div style="display:flex;flex-wrap:wrap;gap:0.5rem;align-items:center;">
       <form method="post" action="/plugins/docker/disk/{{ g.host_id }}/prune" style="margin:0;"
             data-msg="Remove all STOPPED containers on {{ g.host_name|e }}? This cannot be undone."
-            onsubmit="return confirm(this.dataset.msg);">
+            onsubmit="return confirm(this.dataset.msg);">{{ csrf_field() }}
         <input type="hidden" name="target" value="containers">
         <button type="submit" class="btn btn-sm">Prune stopped containers</button>
       </form>
       <form method="post" action="/plugins/docker/disk/{{ g.host_id }}/prune" style="margin:0;"
             data-msg="Remove ALL unused images on {{ g.host_name|e }} (docker image prune -a)? Any image not used by a container is deleted."
-            onsubmit="return confirm(this.dataset.msg);">
+            onsubmit="return confirm(this.dataset.msg);">{{ csrf_field() }}
         <input type="hidden" name="target" value="images">
         <button type="submit" class="btn btn-sm">Prune unused images</button>
       </form>
       <form method="post" action="/plugins/docker/disk/{{ g.host_id }}/prune" style="margin:0;"
             data-msg="Run a full system prune on {{ g.host_name|e }}? Removes stopped containers, unused networks, dangling images and build cache."
-            onsubmit="return confirm(this.dataset.msg);">
+            onsubmit="return confirm(this.dataset.msg);">{{ csrf_field() }}
         <input type="hidden" name="target" value="system">
         <button type="submit" class="btn btn-sm btn-danger">System prune…</button>
       </form>