[Contract] Implement InvoiceContract on Soroban for on-chain invoice state

[JayWebtech]

Overview

Invoices are a roadmap feature. The backend has an InvoiceService but there is no corresponding on-chain contract to track invoice state, prevent double-payment, or provide a verifiable invoice proof.

Technical Requirements

• Create fluxapay/src/invoice.rs Soroban contract.
• Store: invoice_id, merchant_id, amount, currency, due_date, status (Draft/Issued/Paid/Cancelled), payment_id (Option).
• Expose: create_invoice, issue_invoice, mark_paid(payment_id), cancel_invoice, get_invoice.
• Link to PaymentProcessor: mark as Paid when the linked payment is confirmed.
• Emit INVOICE/ISSUED, INVOICE/PAID, INVOICE/CANCELLED events.

[ChukwuemekaP1]

@ChukwuemekaP1 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Hey, I'd like to work on this.

I've written Soroban smart contracts in Rust and understand why on-chain invoice state matters here — without it, there's no trustless proof that an invoice was issued before payment, no double-payment prevention that both parties can verify, and no immutable audit trail that survives a backend compromise.

I'll create fluxapay/src/invoice.rs with the full InvoiceContract — storing invoice_id, merchant_id, amount, currency, due_date, status, and optional payment_id, exposing create_invoice, issue_invoice, mark_paid, cancel_invoice, and get_invoice, linking mark_paid to PaymentProcessor confirmation, and emitting INVOICE/ISSUED, INVOICE/PAID, and INVOICE/CANCELLED events with properly structured payloads. State transition guards will prevent invalid moves like paying a cancelled invoice or issuing an already-paid one.

ETA: 12–24 hours from assignment. Can you assign this to me?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ChukwuemekaP1 to this issue.

[jerrybarry]

@jerrybarry has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Hi, I'd love to work on this issue

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

[drips-wave]

Congratulations, @jerrybarry! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

🧑‍💻 @jerrybarry: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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[jerrybarry]

@JayWebtech please close #88