chore(corp/rih): bootstrap backend application with rouille

Change-Id: I48b0c432a843db5a44b3a3de09f18e1232644a44
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8715
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

corp/rih/backend/src/main.rs

@@ -0,0 +1,8 @@
+use rouille::{Request, Response};
+use std::env;
+
+fn main() {
+    let port = env::var("PORT").unwrap_or_else(|_| /* rihb = */ "7442".to_string());
+    let listen = format!("0.0.0.0:{port}");
+    rouille::start_server(&listen, move |_request| Response::text("hello world"));
+}