feat(forum): add quick publish, captcha and hidden content

Port the reference inline quick-publish composer to the forum home:
title/body inputs, image upload, plate/topic/access/hidden-content
dropdowns and save/publish actions. Wire the site captcha into forum
publishing through a new `forum_post` scene (admin + REST gate), and
turn the hidden-content box into the theme's own visibility shortcodes
(login/comment/vip/payshow) so the existing gate renderer unlocks them.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/admin-contract-report.json

@@ -1,5 +1,5 @@
 {
-    "generated_at": "2026-11-11T08:16:48+00:00",
+    "generated_at": "2026-11-11T08:37:24+00:00",
     "errors": [],
     "warnings": [
         "customPage tools-update section 'tools/update' not in PHP schema (ok if pure operational page)",