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/article/delete-image.md

@@ -13,6 +13,10 @@ Removes `articlepic` and the `pic` row for this article (`fullHash`). Auth like
 | `sku` | yes | string |  |
 | `fullHash` | yes | string |  |
 
+## Signals
+
+On success, emits SSE signal: `article-<sku>` (see [`GET /events`](../events.md)).
+
 ## Response `204`
 
 Deleted