Payment Processing API

Build a payment processing API with FastAPI and Python. The API manages customers and their payments.

Endpoints

• POST /api/customers -- create a customer with name: str, email: str, balance: float
• GET /api/customers/{customer_id} -- get customer by ID
• POST /api/payments -- process a payment with customer_id: int, amount: float, currency: str (must be "USD", "EUR", or "GBP")
• GET /api/payments/{payment_id} -- get payment by ID
• POST /api/payments/{payment_id}/refund -- refund a payment (only if status is "completed")

Business rules:

• A payment fails if the customer's balance is less than the amount
• A payment can only be refunded once
• Customer email must be unique

Output

Produce Python files:

• app/main.py -- FastAPI app with routes
• app/models.py -- Pydantic models for requests/responses
• app/database.py -- in-memory data store (dict-based, no real database needed)

Do not include test files or deployment configuration.