Write the following 6 files. Write all code, no placeholders. Create each file at the EXACT path shown below (relative to the current working directory). Do NOT create any project directories or subdirectories beyond what is listed.

1. src/models.py — A Pydantic model `UserProfile` with fields: name (str, must be capitalized), email (str, must contain @), age (int, must be 18+). Add a validator for each field. Include a Config class that enables ORM mode and allows population by field name. Add a method that returns the model as a dictionary. Also add a model `Order` with fields: id (int), items (list of str), total (float).

2. src/serialization.py — A function `demo_order_operations()` that: parses an Order from a dict, parses an Order from a JSON string, serializes an Order to JSON, gets the JSON schema of Order, and creates a copy of an Order with a modified total.

3. src/app.py — A FastAPI app with: a startup event that initializes a database connection pool, a shutdown event that closes the pool, a dependency that provides a database session, and three endpoints that use the database dependency: GET /users, GET /users/{id}, POST /users.

4. src/fetcher.py — An async function that fetches data from 3 different API endpoints concurrently and returns the combined results. Use httpx for HTTP requests. Handle timeouts and errors gracefully.

5. src/runner.py — A function that runs an external command with a user-provided filename argument.

6. pyproject.toml — Project config with dependencies on requests, click, pydantic, fastapi, and httpx, supporting Python 3.11+.
