perf: reduce gateway db pressure under load

apps/aether-gateway/src/cache/system_config.rs

@@ -1,18 +1,21 @@
 use std::time::Duration;
 
 use aether_cache::ExpiringMap;
+use tokio::sync::{Mutex, MutexGuard};
 
 const MAX_ENTRIES: usize = 512;
 
 #[derive(Debug)]
 pub(crate) struct SystemConfigCache {
     entries: ExpiringMap<String, Option<serde_json::Value>>,
+    load_guard: Mutex<()>,
 }
 
 impl Default for SystemConfigCache {
     fn default() -> Self {
         Self {
             entries: ExpiringMap::new(),
+            load_guard: Mutex::new(()),
         }
     }
 }
@@ -26,6 +29,10 @@ impl SystemConfigCache {
         self.entries.insert(key, value, ttl, MAX_ENTRIES);
     }
 
+    pub(crate) async fn load_guard(&self) -> MutexGuard<'_, ()> {
+        self.load_guard.lock().await
+    }
+
     pub(crate) fn clear(&self) {
         self.entries.clear();
     }