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/delete-image.md

@@ -14,6 +14,10 @@ Auth like GET /category/tree — user must be authorized for the category's comp
 |-------|----------|------|-------------|
 | `id` | yes | integer | Category id |
 
+## Signals
+
+On success, emits SSE signal: `category` (see [`GET /events`](../events.md)).
+
 ## Response `200`
 
 Image removed