Add fediverse social tables, protocols, and implementations

6 new ORM models (remote actors, following, remote posts, local posts,
interactions, notifications), 20 new FederationService methods with
SQL implementations and stubs, WebFinger client, and Alembic migration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

models/__init__.py

@@ -29,4 +29,5 @@ from .container_relation import ContainerRelation
 from .menu_node import MenuNode
 from .federation import (
     ActorProfile, APActivity, APFollower, APInboxItem, APAnchor, IPFSPin,
+    RemoteActor, APFollowing, APRemotePost, APLocalPost, APInteraction, APNotification,
 )