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/history.md

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+# `GET /article/{sku}/history`
+
+Protected. Requires a valid session cookie (`credentials: 'include'`).
+
+Get article history by SKU
+
+Returns `articlehistory` rows for the article, newest first (`date` DESC, `id` DESC).
+Auth like GET by SKU.
+
+
+## Path parameters
+
+| Param | Required | Type | Description |
+|-------|----------|------|-------------|
+| `sku` | yes | string | Article SKU |
+
+## Response `200`
+
+Article history (newest first)
+
+```json
+{
+  "history": [
+    {
+      "id": 42,
+      "date": "2026-08-05T07:15:00.000Z",
+      "text": "Name geändert von \"Tee\" auf \"Grüner Tee\"\nEinheit geändert von \"500g\" auf \"1kg\""
+    },
+    {
+      "id": 41,
+      "date": "2026-08-01T09:00:00.000Z",
+      "text": "Name geändert von \"\" auf \"Tee\""
+    }
+  ]
+}
+```
+
+## Errors
+
+| Status | Description |
+|--------|-------------|
+| 400 | Missing sku — `{"error":"…"}` |
+| 401 | Unauthorized — `{"error":"…"}` |
+| 403 | Not authorized — `{"error":"…"}` |
+| 404 | Article not found — `{"error":"…"}` |
+
+---
+
+_Generated from `@openapi` JSDoc. Do not edit by hand — run `npm run docs:api`._