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/create.md

@@ -15,6 +15,10 @@ Creates a category under an authorized parent.
 }
 ```
 
+## Signals
+
+On success, emits SSE signal: `category` (see [`GET /events`](../events.md)).
+
 ## Response `201`
 
 Created