Minimal ActivityPub model types

[sij411]

Description

Define the minimum ActivityPub-ish model types needed to run mocked Follow and Create(Note) flows.

Proposal

• Implement core types in crates/feder-model
• Add serde (de)serialization
• Add a couple JSON roundtrip tests

Done when

• Can construct/serialize/deserialize: Actor, Follow, Accept, Note, Create(Note)
• Tests cover basic JSON

Out of scope

• Full JSON-LD expansion/validation
• Extra activities (Like/Announce/Undo/etc.)