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/Middleware/SocialPubConfigurations.cs

@@ -11,7 +11,9 @@ using System.Text.Json.Serialization;
 using Microsoft.OpenApi;
 using PrivaPub.Services.ClientToServer.Private;
 using PrivaPub.Services.ClientToServer.Public;
-using PrivaPub.Services.Federation;
+using PrivaPub.Federation.Actors;
+using PrivaPub.Federation.Outbox;
+using PrivaPub.Federation.Inbox;
 
 namespace PrivaPub.Middleware
 {