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/openapi.yaml

@@ -0,0 +1,115 @@
+openapi: 3.0.3
+info:
+  title: CRUD Example Service
+  version: "1.0.0"
+  description: Minimal OpenAPI-first CRUD service with in-memory mock data.
+
+paths:
+  /items:
+    get:
+      operationId: list_items
+      summary: List all items
+      responses:
+        "200":
+          description: List of items
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: "#/components/schemas/Item"
+
+    post:
+      operationId: create_item
+      summary: Create a new item
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: "#/components/schemas/ItemCreate"
+      responses:
+        "201":
+          description: Item created
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/Item"
+
+  /items/{item_id}:
+    get:
+      operationId: get_item
+      summary: Get item by ID
+      parameters:
+        - name: item_id
+          in: path
+          required: true
+          schema:
+            type: integer
+      responses:
+        "200":
+          description: Item found
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/Item"
+
+    put:
+      operationId: update_item
+      summary: Update an item
+      parameters:
+        - name: item_id
+          in: path
+          required: true
+          schema:
+            type: integer
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: "#/components/schemas/ItemCreate"
+      responses:
+        "200":
+          description: Item updated
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/Item"
+
+    delete:
+      operationId: delete_item
+      summary: Delete an item
+      parameters:
+        - name: item_id
+          in: path
+          required: true
+          schema:
+            type: integer
+      responses:
+        "204":
+          description: Item deleted
+
+components:
+  schemas:
+    Item:
+      type: object
+      properties:
+        id:
+          type: integer
+        name:
+          type: string
+        price:
+          type: number
+          format: float
+      required: [id, name, price]
+
+    ItemCreate:
+      type: object
+      properties:
+        name:
+          type: string
+        price:
+          type: number
+          format: float
+      required: [name, price]