docs(data-store): clarify for_type/for_id comment

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/elements/element_data_store.svelte

@@ -278,8 +278,8 @@ async function handle_submit_form(event: Event) {
     }
 
     const api_cfg = untrack(() => $ae_api);
-    // for_type and for_id are parent-context fields set at creation time; never send them on update
-    // (for_id is an integer FK in the DB — sending a string random ID causes a 400 error).
+    // for_type/for_id are creation-time context; never re-send on update.
+    // DB stores for_id as integer; backend resolves and returns the random string — frontend must not send the string back.
     const update_do: key_val = { ...data_store_do };
     delete update_do.for_type;
     delete update_do.for_id;