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/models.py

@@ -0,0 +1,18 @@
+"""
+Pydantic domain models for the CRUD example.
+"""
+
+from pydantic import BaseModel
+
+
+class ItemBase(BaseModel):
+    name: str
+    price: float
+
+
+class ItemCreate(ItemBase):
+    pass
+
+
+class Item(ItemBase):
+    id: int