perf: add 30 second app cache

There used to be an app cache here that would be invalidated but I
think this may have been removed because the invalidation logic
was causing issues. A 30 second cache seems reasonable though, so
let's do that for now.

src/backend/src/helpers.js

@@ -365,6 +365,10 @@ async function refresh_associations_cache(){
     app = app && app[0] ? app[0] : null;
 
     if ( app === null ) return null;
+    
+    kv.set(`apps:uid:${app.uid}`, app, { EX: 30 });
+    kv.set(`apps:name:${app.name}`, app, { EX: 30 });
+    kv.set(`apps:id:${app.id}`, app, { EX: 30 });
 
     // shallow clone because we use the `delete` operator
     // and it corrupts the cache otherwise