[DOCS] Document smart contract storage schema

[dev-fatima-24]

Priority: medium

Description

The contract storage schema (patient → token IDs, token ID → record, issuer → bool) is only described briefly in the README. A detailed schema document helps developers understand data layout and query patterns.

Acceptance Criteria

• All storage keys and their value types are documented
• Key derivation logic is explained
• Maximum storage sizes and cost implications are noted
• Schema document is stored in docs/contract-storage.md
• Document is updated when the schema changes

[rainwaters11]

@rainwaters11 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello! I am highly interested in working on this issue. Please assign it to me.

I understand all the acceptance criteria and can deliver a comprehensive, production-grade schema document inside docs/contract-storage.md.

What I will deliver:

• A clean, tabular inventory of all state variables, data layouts, and explicit type sizes.
• Clear explanations of the contract's key generation and derivation logic for complex mappings.
• An outline highlighting storage slot allocation, maximum sizes, and long-term gas cost implications for modifying these states.
  I look forward to contributing a clean, maintainable documentation update to the VacciChain repository!

ℹ️ Repo Maintainers: To accept this application, review their application or assign @rainwaters11 to this issue.

[drips-wave]

Congratulations, @rainwaters11! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @rainwaters11: 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.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊