A post named by its page is found as by its id

Pixelfed names one of our posts by the address of its page
(/@name/<post id>, the post's url) in its Like, Announce and their Undo,
so its likes were dropped as unknown objects. Before an activity is
handled, such a reference to a post of ours, as its object or the object
of the activity it undoes, is replaced by the post's id. Found by the
pasture's new Pixelfed peer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub.Tests/Support/Harness.cs

@@ -71,7 +71,7 @@ namespace PrivaPub.Tests.Support
 				new BlockHandler(Db, Local)
 			};
 			((AnnounceHandler)Handlers.First(h => h is AnnounceHandler)).Relays = Handlers;
-			Processor = new InboxProcessor(Remote, Handlers, NullLogger<InboxProcessor>.Instance, Ledger);
+			Processor = new InboxProcessor(Remote, Handlers, NullLogger<InboxProcessor>.Instance, Ledger, Local);
 			Follows = new FollowService(Db, Local, Remote, Delivery, new KeyLocalizer<GenericRes>(), NullLogger<FollowService>.Instance);
 			Content = new ContentRenderer(Local, Remote);
 			Media = new MediaService(new StaticOptions<MediaOptions>(new MediaOptions { Root = Path.Combine(Path.GetTempPath(), $"privapub-media-{Guid.NewGuid():N}") }),