Convert low-risk entities from ArtObject to IPF (Category, Report, Rate, Digest)

[fiammybe]

Validate the migration approach by converting simpler models first before touching Forum, Topic, and Post.

Checklist:

• Migrate Category, Report, Rate, and Digest to IPF objects/handlers
• Update all usages to load via IPF handler
• Update admin CRUD where possible for these entities to use IPF
• Add/adjust tests and verify the admin UI still works
• Completely remove Art from these entities