[P2] Audit: Build Merkle tree of event commitments for batch proofs

[Calebux]

Parent Issue

#827

Summary

Organize event commitments into a Merkle tree so users can prove membership of individual events without revealing the full log.

Details

• Each new commitment is appended as a leaf to a Merkle tree
• Contract stores only the Merkle root (updated incrementally)
• User can generate a Merkle proof for any leaf (event) they know
• Verifier checks: MerkleVerify(root, leaf, proof) == true
• Tree is append-only (no deletions)

Affected Code

• New: contracts/contracts/subscription_logging/src/merkle.rs
• shared/src/crypto/merkle.ts — client-side proof generation
• backend/src/services/blockchain-service.ts — maintain local copy of tree

Acceptance Criteria

• Merkle root is updated on-chain with each new commitment
• User can generate proof for any past event
• Proof verification works on-chain and off-chain
• Tree handles 10,000+ leaves efficiently

[Chi-nenye1975]

@Chi-nenye1975 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer, I'd love to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Chi-nenye1975 to this issue.

[drips-wave]

Congratulations, @Chi-nenye1975! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @Chi-nenye1975: 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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