authorized fetch: let /@instance.actor through

this is probably never actually used, but it still looks like a good
idea (also, FireFish does it)

thanks @ShittyKoper for noticing!

packages/backend/src/server/ActivityPubServerService.ts

@@ -131,6 +131,7 @@ export class ActivityPubServerService {
 			const instanceActor = await this.instanceActorService.getInstanceActor();
 
 			if (userId === instanceActor.id) return false;
+			if (userId === instanceActor.username) return false;
 		}
 
 		let signature;