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/dashboard/share.html

@@ -32,7 +32,7 @@
             </div>
           </div>
           <form method="post" action="/d/{{ dashboard.id }}/share/revoke/{{ token.id }}"
-                style="margin:0;" onsubmit="return confirm('Revoke this share link?')">
+                style="margin:0;" onsubmit="return confirm('Revoke this share link?')">{{ csrf_field() }}
             <button type="submit" class="btn btn-danger btn-sm">Revoke</button>
           </form>
         </div>
@@ -50,7 +50,7 @@
   <div>
     <div class="section-title" style="margin-bottom:0.75rem;">Create Share Link</div>
     <div class="card" style="margin-bottom:0;">
-      <form method="post" action="/d/{{ dashboard.id }}/share/create">
+      <form method="post" action="/d/{{ dashboard.id }}/share/create">{{ csrf_field() }}
         <div class="form-group" style="margin-bottom:0.75rem;">
           <label>Label <span style="color:var(--text-muted);font-size:0.78rem;">(optional)</span></label>
           <input type="text" name="label" placeholder="e.g. Public display screen">