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/events.md

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+# `GET /events`
+
+Protected. Requires a valid session cookie (`credentials: 'include'`).
+
+Subscribe to update signals (SSE)
+
+Opens a Server-Sent Events stream for the authenticated user's
+**selected company**. Clients receive named signals when another
+request successfully mutates shared data for that company
+(including the same browser's other tabs).
+
+Use `EventSource` with credentials so the session cookie is sent:
+
+```js
+const es = new EventSource('https://…/api/events', { withCredentials: true });
+es.addEventListener('category', () => { refetchCategoryTree(); });
+es.addEventListener('articles', () => { refetchArticleIndex(); });
+es.addEventListener('article-42000045', () => { refetchArticle('42000045'); });
+```
+
+Keep-alive comments (`: keepalive`) are sent roughly every 25s.
+Reconnect after changing the selected company (the stream is bound
+to the company at connect time).
+
+## Signals
+
+| Signal | Emitted after successful |
+|--------|--------------------------|
+| `category` | `POST /category`, `PATCH /category/{id}`, `PUT /category/order`, `DELETE /category/{id}`, `POST /category/{id}/image`, `DELETE /category/{id}/image` |
+| `articles` | `POST /article`, `PATCH /article/{sku}`, `DELETE /article/{sku}` |
+| `article-<sku>` | `PUT /article/{sku}/weight`, `PUT /article/{sku}/prices`, `PUT /article/{sku}/eans`, `PUT /article/{sku}/categories`, `PUT /article/{sku}/brand`, `PUT /article/{sku}/images/order`, `POST /article/{sku}/image`, `DELETE /article/{sku}/image/{fullHash}` |
+
+Signals are only sent when the mutation actually succeeds (2xx).
+Failed or no-op validation errors do not emit. The `article-<sku>`
+event name uses the concrete SKU (e.g. `article-42000045`).
+
+
+## Response `200`
+
+SSE stream (`text/event-stream`). Each signal is an SSE event
+whose name is the signal id and whose data is `{}`.
+
+
+## Errors
+
+| Status | Description |
+|--------|-------------|
+| 400 | No company selected — `{"error":"…"}` |
+| 401 | Unauthorized — `{"error":"…"}` |
+
+---
+
+_Generated from `@openapi` JSDoc. Do not edit by hand — run `npm run docs:api`._