Convert Topic model/handler to IPF

[fiammybe]

Migrate Topic to IPF while preserving topic navigation, moderation, and synchronization behavior.

Checklist:

• Refactor Topic to IPF
• Refactor IforumTopicHandler to IPF
• Migrate approve/getByPost/navigation/post-list/synchronization to helpers if possible
• Regression check: view topic, navigation, moderation, and counters
• Ensure Art is not loaded/required