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.md

@@ -12,6 +12,10 @@ Deletes a category the user is authorized for (not the company root).
 |-------|----------|------|-------------|
 | `id` | yes | integer | Category id |
 
+## Signals
+
+On success, emits SSE signal: `category` (see [`GET /events`](../events.md)).
+
 ## Response `204`
 
 Deleted