feat(scaffold): add model-backed CRUD service template

Provides a complete OpenAPI-first CRUD example with a Pydantic
model layer, explicit runtime semantics, and integration tests
to support developer onboarding and real-world service structure.

openapi_first/templates/model_app/data.py

@@ -0,0 +1,44 @@
+"""
+In-memory data store using Pydantic models.
+
+This module is NOT thread-safe and is intended for demos and scaffolds only.
+"""
+
+from typing import Dict
+
+from models import Item, ItemCreate
+
+_items: Dict[int, Item] = {
+    1: Item(id=1, name="Apple", price=0.5),
+    2: Item(id=2, name="Banana", price=0.3),
+}
+
+_next_id = 3
+
+
+def list_items() -> list[Item]:
+    return list(_items.values())
+
+
+def get_item(item_id: int) -> Item:
+    return _items[item_id]
+
+
+def create_item(payload: ItemCreate) -> Item:
+    global _next_id
+    item = Item(id=_next_id, **payload.model_dump())
+    _items[_next_id] = item
+    _next_id += 1
+    return item
+
+
+def update_item(item_id: int, payload: ItemCreate) -> Item:
+    if item_id not in _items:
+        raise KeyError(item_id)
+    item = Item(id=item_id, **payload.model_dump())
+    _items[item_id] = item
+    return item
+
+
+def delete_item(item_id: int) -> None:
+    del _items[item_id]