Implement lazy storage initialisation to reduce per-invoice rent cost

[Kingsman-99]

Description

Every invoice currently initialises all storage keys (payment map, contributor count, etc.) at creation, incurring rent for entries that may never be used. This issue converts those entries to lazily-initialised storage using env.storage().has() checks before reads, deferring rent until first write.

Technical Context

Audit all create_invoice storage writes in lib.rs and identify entries that are only populated after the first pay_invoice call (e.g. payment_map_key, contributor_count_key, payer_window_key). Replace unconditional writes with lazy get_or_default helpers. Update read paths to handle the absent case. Measure storage slot reduction using a test that compares slot counts before and after.

Acceptance Criteria

• create_invoice writes only the minimum required storage entries (invoice struct, status key)
• All lazily-initialised entries are correctly handled when absent (return default, not panic)
• A test asserts the number of storage entries written per create_invoice is ≤ N (document N)
• All existing payment and release flows continue to pass without regression
• cargo test --test integration runs without error on the refactored storage model
• All CI checks (cargo test, cargo clippy, cargo fmt) pass and the branch has no merge conflicts

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[adeboladee]

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Hello Maintainer, Please I would like to work on this issue

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[Anadudev]

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Hi, maintainer, I would like to work on this issue.

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[numdinkushi]

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Hi, I would like to jump on this

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[williamsokperez-arch]

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

i would like to work on this issue

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[drips-wave]

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

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[drips-wave]

This issue has been completed by @williamsokperez-arch as part of the Stellar Wave Program's 7th Wave 🥳

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