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/add-image.md

@@ -20,6 +20,10 @@ Auth like GET /category/tree — user must be authorized for the category's comp
 
 Raw binary (`image/avif`).
 
+## Signals
+
+On success, emits SSE signal: `category` (see [`GET /events`](../events.md)).
+
 ## Response `201`
 
 Image stored and linked