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/update.md

@@ -23,6 +23,10 @@ Patches article fields. At least one of `name`, `unitamount`, `unitSi` required.
 }
 ```
 
+## Signals
+
+On success, emits SSE signal: `articles` (see [`GET /events`](../events.md)).
+
 ## Response `200`
 
 Updated article detail