Emit PaymentMethod attach and detach webhooks

[neilberkman]

Summary

• Emit payment_method.attached after POST /v1/payment_methods/:id/attach successfully updates the PaymentMethod.
• Emit payment_method.detached after POST /v1/payment_methods/:id/detach successfully clears the customer.
• Add collection-mode coverage for both webhook events and a TestClient.detach_payment_method/1 helper.

Validation

• mise exec -- mix test test/paper_tiger/webhook_collection_test.exs test/paper_tiger/resources/payment_method_test.exs
• mise exec -- mix test
• mise exec -- mix format --check-formatted
• git diff --check