feat: expand ActivityPub protocol documentation

Expanded protocols/activitypub.md from 176 to 425 lines:
- Detailed actor types and properties
- Activity types with side effects table
- Addressing patterns (to/cc/bcc)
- HTTP Signatures deep dive with verification flow
- Collections and pagination
- Common federation issues and solutions
- Follow flow diagram

Total pages: 116

Author: melvincarvalho