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/set-weight.md

@@ -22,6 +22,10 @@ Returns the same shape as GET by SKU.
 }
 ```
 
+## Signals
+
+On success, emits SSE signal: `article-<sku>` (see [`GET /events`](../events.md)).
+
 ## Response `200`
 
 Updated article detail