Implement ActivityPub discovery and public object endpoints

[sij411]

Description

Make the configured local actor discoverable and expose the public ActivityPub resources required by a minimal single-user server.

Scope

• Implement WebFinger for the configured actor.
• Serve the local actor document with appropriate ActivityPub content types.
• Expose public local objects or notes available from runtime storage.
• Return stable canonical IDs and URLs derived from configuration.
• Serialize responses through feder-vocab rather than hand-building protocol JSON.

Done when

• WebFinger resolves the configured account to its actor URL.
• The actor endpoint returns a valid typed ActivityPub actor document.
• A stored public Note can be retrieved by its canonical URL.
• Route and content-negotiation tests cover success and basic error cases.

Out of scope

• Inbox processing.
• HTTP Signature verification.
• Remote actor fetching and outgoing delivery.