fix(appservice): Create user on authentication for existing appservices

Ensures appservice users are created during authentication requests,
fixing the issue for appservices registered before the initial fix.
This allows existing appservices to work without re-registration.

src/api/router/auth.rs

@@ -220,6 +220,10 @@ async fn auth_appservice(
 		return Err!(Request(Exclusive("User is not in namespace.")));
 	}
 
+	if !services.users.exists(&user_id).await {
+		services.users.create(&user_id, None)?;
+	}
+
 	Ok(Auth {
 		origin: None,
 		sender_user: Some(user_id),