Add article history by SKU and SSE update signals for live clients.

Expose getHistoryBySku as GET /article/:sku/history, and document mutation-emitted company signals via GET /events.

docs/api/routes/category/order.md

@@ -24,6 +24,10 @@ to its index `0..n-1` in one transaction.
 }
 ```
 
+## Signals
+
+On success, emits SSE signal: `category` (see [`GET /events`](../events.md)).
+
 ## Response `200`
 
 New sibling order