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>

assets/admin/src/schema/sections/permissions/index.js

@@ -84,6 +84,7 @@ export const permissionsSchemas = {
               { label: '资源下载', value: 'download' },
               { label: '每日签到', value: 'signin' },
               { label: '前台投稿', value: 'front_submit' },
+              { label: '论坛发帖', value: 'forum_post' },
               { label: '打赏用户', value: 'tip_user' },
               { label: '提交工单', value: 'ticket_create' },
               { label: '回复工单', value: 'ticket_reply' },