Build forward invoice chain helper

[Kingsman-99]

Labels: complexity: high

Add getForwardChain(invoiceId) that resolves the forward_invoice_id chain for an invoice, returning the full list of invoices that leftover funds would cascade through at release.

Acceptance criteria:

• Returns { id, status, forwardTo }[] in cascade order
• Detects and errors on circular forward chains
• Returns single-element array when no forwarding is configured
• Max chain depth of 10 before throwing ForwardChainTooDeepError

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[kokobutter-web]

@kokobutter-web has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I'm a full-stack developer and I'd like to take this issue.
I understand the requirements and will follow the existing project structure, keep the implementation clean, and include proper tests where applicable.

ETA: 1–3 days (depending on scope)

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Congratulations, @kokobutter-web! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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