Add creator-set minimum contributor count before release

[Kingsman-99]

Description

Some fundraising invoices should only release if a minimum number of unique contributors participated, preventing a single whale from silently funding an invoice that was intended for broad community participation. This issue adds min_contributors: Option<u32> to invoices with enforcement at release time.

Technical Context

Add min_contributors: Option<u32> to the Invoice struct in types.rs and a contributor_count_key(invoice_id) -> u32 storage entry in lib.rs. Increment the counter in pay_invoice only for first-time payers on that invoice (check payment_exists_key(invoice_id, payer)). In release_invoice, if min_contributors is set and contributor_count < min_contributors, panic with InsufficientContributors.

Acceptance Criteria

• create_invoice accepts optional min_contributors
• Contributor count is incremented at most once per unique payer per invoice
• release_invoice panics with InsufficientContributors if count below threshold
• get_contributor_count(env, invoice_id) -> u32 view function exposed
• Invoices without min_contributors release with no count check (existing behaviour)
• Tests: exact count, under count blocked, over count passes, same payer multi-payment counts once
• All CI checks (cargo test, cargo clippy, cargo fmt) pass and the branch has no merge conflicts

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[altbrightt]

@altbrightt has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi Maintainer,

I’d be happy to work on this issue. I’ve reviewed the requirements and have a clear understanding of the task. If possible, please assign it to me so I can get started. Thank you.

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[olaayoade91-byte]

@olaayoade91-byte has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

hello maintainer please i would like to work on this please assign me

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[farouqyakubu6-creator]

@farouqyakubu6-creator has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Allow me to professionally implement this task with precision and accuracy.

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[felladaniel36-hash]

@felladaniel36-hash has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I'd like to claim this issue. I have reviewed the details and can begin working on a fix right away. Please assign it to me if it's still available. Thanks!

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[drips-wave]

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

🧑‍💻 @altbrightt: 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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