Add unit API and article unitamount/unitSi patch.

Units use the selected company; article PATCH accepts name/unitamount/unitSi.

docs/api/routes/unit/upsert.md

@@ -0,0 +1,44 @@
+# `PUT /unit`
+
+Protected. Requires a valid session cookie (`credentials: 'include'`).
+
+Upsert unit
+
+Insert or update unit by `si` for the selected company. Upserts `unittext` for the company language.
+`basesifactor` must be finite and `> 0`.
+
+
+## Body (JSON)
+
+```json
+{
+  "si": "l",
+  "basesi": "l",
+  "basesifactor": 1,
+  "symbol": "L",
+  "name": "Liter"
+}
+```
+
+## Response `200`
+
+Upserted unit
+
+```json
+{
+  "unit": { … }
+}
+```
+
+## Errors
+
+| Status | Description |
+|--------|-------------|
+| 400 | Invalid unit or no company selected — `{"error":"…"}` |
+| 401 | Unauthorized — `{"error":"…"}` |
+| 403 | Not authorized — `{"error":"…"}` |
+| 404 | Company not found — `{"error":"…"}` |
+
+---
+
+_Generated from `@openapi` JSDoc. Do not edit by hand — run `npm run docs:api`._