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

PrivaPub/Services/ClientToServer/Private/IPrivateAvatarUsersService.cs

@@ -7,8 +7,8 @@ using PrivaPub.ClientModels.User;
 using PrivaPub.ClientModels.User.Avatar;
 using PrivaPub.Models.User;
 using PrivaPub.Resources;
-using PrivaPub.Services.Federation;
 using PrivaPub.StaticServices;
+using PrivaPub.Federation.Actors;
 
 namespace PrivaPub.Services.ClientToServer.Private
 {