feat(templates): add OpenAPI-first CRUD app scaffold with mock data

- include CRUD OpenAPI spec
- add in-memory mock data store
- implement OpenAPI-bound route handlers
- provide runnable FastAPI bootstrap
- include end-to-end integration test

openapi_first/templates/crud_app/data.py

@@ -0,0 +1,41 @@
+"""
+In-memory mock data store for CRUD example.
+
+This module intentionally avoids persistence and concurrency guarantees.
+It is suitable for demos, tests, and scaffolding only.
+"""
+
+from typing import Dict
+
+_items: Dict[int, dict] = {
+    1: {"id": 1, "name": "Apple", "price": 0.5},
+    2: {"id": 2, "name": "Banana", "price": 0.3},
+}
+
+_next_id = 3
+
+
+def list_items():
+    return list(_items.values())
+
+
+def get_item(item_id: int):
+    return _items[item_id]
+
+
+def create_item(payload: dict):
+    global _next_id
+    item = {"id": _next_id, **payload}
+    _items[_next_id] = item
+    _next_id += 1
+    return item
+
+
+def update_item(item_id: int, payload: dict):
+    item = {"id": item_id, **payload}
+    _items[item_id] = item
+    return item
+
+
+def delete_item(item_id: int):
+    del _items[item_id]