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/send_leave.rs

@@ -1,7 +1,7 @@
 #![allow(deprecated)]
 
 use axum::extract::State;
-use conduwuit::{Err, Result, err, matrix::event::gen_event_id_canonical_json};
+use conduwuit::{Err, Result, debug_warn, err, matrix::event::gen_event_id_canonical_json};
 use conduwuit_service::Services;
 use futures::FutureExt;
 use ruma::{
@@ -50,6 +50,19 @@ async fn create_leave_event(
 		return Err!(Request(NotFound("Room is unknown to this server.")));
 	}
 
+	if !services
+		.rooms
+		.state_cache
+		.server_in_room(services.globals.server_name(), room_id)
+		.await
+	{
+		debug_warn!(
+			origin = origin.as_str(),
+			"Refusing to serve backfill for room we aren't participating in"
+		);
+		return Err!(Request(NotFound("This server is not participating in that room.")));
+	}
+
 	// ACL check origin
 	services
 		.rooms