Convert Forum model/handler to IPF and separate display logic

[fiammybe]

Migrate Forum off Art and reduce the amount of display/business logic mixed into the handler.

Checklist:

• Refactor Forum to extend icms_ipf_Object
• Convert IforumForumHandler to IPF
• Extract forum listing (display builder), moderator detection, and synchronization logic to helpers/services
• Keep handler signatures for public APIs (for now)
• Validate forum pages and admin UI still function
• Forum-related classes should have NO Art dependencies