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/main.py

@@ -0,0 +1,8 @@
+from openapi_first.app import OpenAPIFirstApp
+import routes
+
+app = OpenAPIFirstApp(
+    openapi_path="openapi.yaml",
+    routes_module=routes,
+    title="Model CRUD Example Service",
+)