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/routes.py

@@ -0,0 +1,50 @@
+"""
+CRUD route handlers bound via OpenAPI operationId.
+
+These handlers explicitly control HTTP status codes to ensure
+runtime behavior matches the OpenAPI contract.
+"""
+
+from fastapi import Response, HTTPException
+
+from data import (
+    list_items as _list_items,
+    get_item as _get_item,
+    create_item as _create_item,
+    update_item as _update_item,
+    delete_item as _delete_item,
+)
+
+
+def list_items():
+    return _list_items()
+
+
+def get_item(item_id: int):
+    try:
+        return _get_item(item_id)
+    except KeyError:
+        raise HTTPException(status_code=404, detail="Item not found")
+
+
+def create_item(payload: dict, response: Response):
+    item = _create_item(payload)
+    response.status_code = 201
+    return item
+
+
+def update_item(item_id: int, payload: dict):
+    try:
+        return _update_item(item_id, payload)
+    except KeyError:
+        raise HTTPException(status_code=404, detail="Item not found")
+
+
+def delete_item(item_id: int, response: Response):
+    try:
+        _delete_item(item_id)
+    except KeyError:
+        raise HTTPException(status_code=404, detail="Item not found")
+
+    response.status_code = 204
+    return None