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/PostsService.cs

@@ -7,13 +7,15 @@ using PrivaPub.ClientModels.Post;
 using PrivaPub.Models.Federation;
 using PrivaPub.Models.Group;
 using PrivaPub.Resources;
-using PrivaPub.Services.Federation;
 using PrivaPub.StaticServices;
 
 using System.Text.Json.Nodes;
 
 using DmPostEntity = PrivaPub.Models.Post.DmPost;
 using PostEntity = PrivaPub.Models.Post.Post;
+using PrivaPub.Federation.Actors;
+using PrivaPub.Federation.Rendering;
+using PrivaPub.Federation.Outbox;
 
 namespace PrivaPub.Services
 {