Milestone 1: Enhance Signature Handling for Actors

We will refactor the signature handling architecture to support Repository and Organization actors in addition to the currently supported Instance and User actors. This architectural refactoring is essential for enabling repositories and organizations to participate in federated activities with proper authentication and authorization. We will document our decisions in an Architecture Decision Record and deliver the refactored architecture as a pull request to forgejo-contrib/federation. This work establishes the foundation for more complex federation features by ensuring all actor types can securely communicate across instances.

A.C.:

Architecture is updated
SignatureService is cleaned up & we've migrated existing key material in to the new structure

Deliveries:

Architecture
PR about http signature details: #57 (merged)
PR about ID handling / normalization: #82 (merged)
PR about having federation keys in own table: #73 (merged)
PR about simplified signature verification: #84
PR about changing the lifecycle to reduce normalization issues: #85
forgejo code
Simplify signature validation to be the least common denominator: forgejo/forgejo#10189 (merged)
Add support for RFC9421 signatures (which is implemented by Mastodon, and may be implemented by GoToSocial in near future): forgejo/forgejo#10206 - added examples for api.ini configuration of RFC 9421 signatures: forgejo/docs#1672
Handle federated key material as first class citizen instead of attaching them to their actors: forgejo/forgejo#10074
simplified signature verification: 1. federation-dev: meissa/forgejo#38 2. forgejo: waiting for PR
refactor model.forgefed -> model.federation_host 1. federation-dev: meissa/forgejo#39 2. forgejo: waiting for PR
(wip) refactor federation: use optional return types: forgejo/forgejo#10329