Build invoice payment notification hooks

[Kingsman-99]

Label: complexity: high
Points: 200

Description

External contracts need to react to every payment event for composable DeFi workflows. This issue adds payment_hook: Option<Address> to create_invoice() — after every successful pay(), the contract calls on_payment(invoice_id, payer, amount) on the registered hook contract.

Technical Context

Involves types.rs (add payment_hook: Option<Address>) and lib.rs pay(). After updating invoice.funded and before auto-release check, if payment_hook is Some, invoke it via cross-contract call with (invoice_id, payer, amount). Hook failure must not revert the payment.

Acceptance Criteria

• create_invoice() accepts optional payment_hook
• Hook called after every successful pay()
• Hook failure does not revert the payment transaction
• payment_hook = None produces identical behaviour to current implementation
• Test mocks hook contract and verifies it is called with correct arguments
• All existing cargo tests pass
• cargo clippy passes with zero warnings

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[levi0005]

@levi0005 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi maintainer, i would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @levi0005 to this issue.

[drips-wave]

Congratulations, @levi0005! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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