Add customergroup and entity HTTP APIs with address and contact routes.

Expose store-backed CRUD for customer groups, entities, nested addresses, and contacts, matching the documented dashdbschema contracts.

docs/api/routes/customergroup/create.md

@@ -0,0 +1,41 @@
+# `POST /customergroup`
+
+Protected. Requires a valid session cookie (`credentials: 'include'`).
+
+Create customer group
+
+Creates a customer group for the selected company.
+
+## Body (JSON)
+
+```json
+{
+  "name": "Kunden"
+}
+```
+
+## Response `201`
+
+Created customer group
+
+```json
+{
+  "customergroup": {
+    "id": 1,
+    "name": "…"
+  }
+}
+```
+
+## Errors
+
+| Status | Description |
+|--------|-------------|
+| 400 | Name required 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`._