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.

src/api/openapi.js

@@ -88,6 +88,17 @@
  *       type: object
  *       description: Full article payload (unit, brand, categoryIds, photos, eans, prices, …)
  *       additionalProperties: true
+ *     ArticleHistoryEntry:
+ *       type: object
+ *       required: [id, date, text]
+ *       properties:
+ *         id:
+ *           type: integer
+ *         date:
+ *           type: string
+ *           format: date-time
+ *         text:
+ *           type: string
  *     Brand:
  *       type: object
  *       required: [id, name]