refactor(admin): migrate forum management to new architecture
Migrate the forum admin mini-module (legacy includes/forum dispatcher, 8 modes)
to controllers under /admin/forum: dashboard, settings (rights>=9), structure
CRUD (categories/sections with slug generation and delete flows that move
subsections/topics or cascade-delete content), and hidden topics/posts with
author/section filters and rights>=9 bulk purge. Reuse forum module models
(ForumSection/Topic/Message/File/Vote/Unread); add ForumAdmin/ForumConfig/
ForumStructure/HiddenForum repositories, use cases, slug generator and hidden
row mappers. Convert all ?mod= GET mutations to POST forms with CSRF and PRG.
Remove includes/forum.php and includes/forum/, drop the dispatcher entry, and
point both sidebars at /admin/forum.