perf: Don't handle expensive requests for rooms we aren't in

Mostly borrowed from dendrite:

https://github.com/element-hq/dendrite/blob/a042861/federationapi/routing/routing.go#L601

src/api/server/make_join.rs

@@ -1,7 +1,9 @@
 use std::borrow::ToOwned;
 
 use axum::extract::State;
-use conduwuit::{Err, Error, Result, debug, debug_info, info, matrix::pdu::PduBuilder, warn};
+use conduwuit::{
+	Err, Error, Result, debug, debug_info, debug_warn, info, matrix::pdu::PduBuilder, warn,
+};
 use conduwuit_service::Services;
 use futures::StreamExt;
 use ruma::{
@@ -30,6 +32,18 @@ pub(crate) async fn create_join_event_template_route(
 	if !services.rooms.metadata.exists(&body.room_id).await {
 		return Err!(Request(NotFound("Room is unknown to this server.")));
 	}
+	if !services
+		.rooms
+		.state_cache
+		.server_in_room(services.globals.server_name(), &body.room_id)
+		.await
+	{
+		debug_warn!(
+			origin = body.origin().as_str(),
+			"Refusing to serve make_join for room we aren't participating in"
+		);
+		return Err!(Request(NotFound("This server is not participating in that room.")));
+	}
 
 	if body.user_id.server_name() != body.origin() {
 		return Err!(Request(BadJson("Not allowed to join on behalf of another server/user.")));