Move the federation code under Federation/, namespaces only

Services/Federation and Controllers/ServerToServer become
Federation/{Actors,Signing,Inbox,Outbox,Rendering,Controllers}, the first step
of the roadmap's layout. No type, route or behaviour changes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

CLAUDE.md

@@ -73,13 +73,16 @@ PrivaPub/                         ASP.NET Core Web API, net10.0
   Middleware/SocialPubConfigurations.cs   every DI registration (auth, federation, services, swagger, CORS)
   Controllers/ClientToServer/     /clientapi/*: RootUser (signup/login/invitations/recovery), PrivateAvatar,
                                   Group, Post (posts + DMs), Admin, Data
-  Controllers/ServerToServer/     PeasantsController (actor, outbox, followers, following, posts, inboxes),
+  Federation/
+    Controllers/                  PeasantsController (actor, outbox, followers, following, posts, inboxes),
                                   WellKnownController (webfinger, nodeinfo), UsersController (redirect)
+    Actors/                       LocalActorService (LocalActor, Keys), RemoteActorService (signed fetch,
+                                  WebFinger, ForeignAvatar cache)
+    Signing/                      HttpSignatures (draft-cavage sign/verify)
+    Inbox/                        InboxService (Follow/Undo/Create/Delete/Update)
+    Outbox/                       DeliveryService + DeliveryWorker
+    Rendering/                    ActivityPubRenderer (JsonObject builders)
   Services/                       RootUsersService, GroupUsersService, PostsService, AppConfigurationService, …
-  Services/Federation/            LocalActorService (LocalActor, Keys), HttpSignatures (draft-cavage),
-                                  RemoteActorService (signed fetch, WebFinger, ForeignAvatar cache),
-                                  ActivityPubRenderer (JsonObject builders), DeliveryService + DeliveryWorker,
-                                  InboxService (Follow/Undo/Create/Delete/Update)
   Models/                         Mongo entities: User/, Group/, Post/, Federation/, AppConfiguration
   StaticServices/                 DbEntities (Find<T> accessors), AuthTokenManager (JWT), PasswordHasher
   Data/InitDb.cs                  first-run seeding (languages)