Contracts: Add on-chain campaign metadata (name, description, image URI) to campaign contract

[CelestinaBeing]

Summary

The campaign contract stores only operational config (active flag, time window, cap, Merkle root). Campaign name, description, and an optional image_uri are stored only in the off-chain backend DB — meaning on-chain verification of campaign identity is impossible.

Problem

• Users cannot verify a campaign's metadata from the contract alone
• Forks of the backend can display arbitrary metadata for any contract ID
• No trustless way to confirm what a campaign is about before registering

Acceptance Criteria

• Add set_metadata(env, admin, nonce, name: Symbol, description: String, image_uri: String) admin function
• Add get_metadata(env) -> (Symbol, String, String) view function
• Store in instance storage; emit a metadata event on update
• Keep name as Symbol (max 32 chars), description and image_uri as String (max 256 chars each) — reject with InvalidMetadata error if too long
• Add unit tests: set and get, unauthorized set, string length boundary
• Update backend/src/jobs/eventIndexer.js (issue Backend: Implement Soroban event indexer worker #283) to index the metadata event and sync with DB
• Update docs/ARCHITECTURE_OVERVIEW.md

References

• contracts/campaign/src/lib.rs
• contracts/rewards/src/lib.rs:81 — metadata() view pattern to follow

[Valreb001]

@Valreb001 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Good day maintainer, please i would like to work on this issue, assign me please!!!

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Valreb001 to this issue.

[prodbycorne]

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

I would like to work to on this issue

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

[drips-wave]

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

🧑‍💻 @prodbycorne: 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 🌊

[drips-wave]

This issue has been completed by @prodbycorne as part of the Stellar Wave Program's 5th Wave 🥳

😎 @prodbycorne: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

🧑‍💻 Repo maintainers: How'd the contributor do? Leave a review to share your experience working with them.