webhook: compare secret in constant time

payload.secret != expected_secret short-circuits on the first
differing byte, which leaks timing information about how many
leading bytes of a guess are correct. Use subtle::ConstantTimeEq
instead.

src/webhook.rs

@@ -1,5 +1,6 @@
 use axum::{extract::State, http::StatusCode, response::IntoResponse};
 use serde::Deserialize;
+use subtle::ConstantTimeEq;
 
 use crate::AppState;
 
@@ -28,7 +29,9 @@ pub async fn receive(
             .into_response();
     };
 
-    if payload.secret != expected_secret {
+    let secret_matches = payload.secret.as_bytes().ct_eq(expected_secret.as_bytes());
+
+    if !bool::from(secret_matches) {
         tracing::warn!("received webhook with invalid secret");
 
         return (StatusCode::UNAUTHORIZED, "invalid webhook secret").into_response();