Add customergroup bulk reorder and list editable flag.

Expose PUT /customergroup/order for exact-set sort updates, keep create/update name-only, and mark POS/Lieferanten as non-editable in list responses.

docs/api/routes/customergroup/create.md

@@ -5,6 +5,9 @@ Protected. Requires a valid session cookie (`credentials: 'include'`).
 Create customer group
 
 Creates a customer group for the selected company.
+Appends with the next `sort` index. Reserved names `POS` and `Lieferanten`
+are not allowed (`Name not allowed`).
+
 
 ## Body (JSON)
 
@@ -22,7 +25,8 @@ Created customer group
 {
   "customergroup": {
     "id": 1,
-    "name": "…"
+    "name": "…",
+    "sort": 1
   }
 }
 ```
@@ -31,7 +35,7 @@ Created customer group
 
 | Status | Description |
 |--------|-------------|
-| 400 | Name required or no company selected — `{"error":"…"}` |
+| 400 | Name required, name not allowed, or no company selected — `{"error":"…"}` |
 | 401 | Unauthorized — `{"error":"…"}` |
 | 403 | Not authorized — `{"error":"…"}` |
 | 404 | Company not found — `{"error":"…"}` |