Stripepay Backend: Implement a Simplified Payment Processing Backend

Question

## Problem

Build a simplified payment backend with the following operations:

1. `create_customer(name, email)` -> `customer_id`
2. `add_payment_method(customer_id, card_number, expiry)` -> `method_id` (validate card format)
3. `charge(customer_id, method_id, amount_cents)` -> `{"status": "success" | "failed", "charge_id": str}`
4. `get_charges(customer_id)` -> list of charge records

```python
class PaymentBackend:
    def create_customer(self, name: str, email: str) -> str: ...
    def add_payment_method(self, customer_id: str, card_number: str, expiry: str) -> str: ...
    def charge(self, customer_id: str, method_id: str, amount_cents: int) -> dict: ...
    def get_charges(self, customer_id: str) -> list: ...
```

**Example:**
```
b = PaymentBackend()
cid = b.create_customer("Alice", "alice@x.com")   # -> "cust_001"
mid = b.add_payment_method(cid, "4111111111111111", "12/26")
result = b.charge(cid, mid, 4999)
# -> {"status": "success", "charge_id": "ch_001"}
b.get_charges(cid)
# -> [{"charge_id": "ch_001", "amount": 4999, "status": "success"}]
```

## Follow-ups

1. How do you implement idempotency so retrying a failed request does not double-charge?
2. What data should you never store in plain text, and how would you tokenize card numbers?
3. How would you add webhook support to notify the merchant of charge status asynchronously?
4. How would you model refunds and partial refunds against existing charges?

Stripepay Backend: Implement a Simplified Payment Processing Backend

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