Add /api/predictions claim flow that submits the on-chain claim tx

[greatest0fallt1me]

Description

This is a backend issue for the GrantFox campaign. Endpoint that, after market resolution, builds and submits a Soroban claim tx for the authenticated user and records the receipt.

Requirements and Context

• Validates resolution status
• Builds Soroban tx via stellar-sdk
• Persists tx_hash in predictions
• Idempotent via Idempotency-Key
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b feature/predictions-claim

2. Implement changes
   • src/routes/predictions/claim.ts (new)
   • src/services/claimService.ts (new)
   • src/db/schema.ts
3. Test and commit
   • Run the repo's standard test suite and lint
   • Cover edge cases; include output in the PR

Example commit message

feat: predictions claim flow

Acceptance Criteria

• Validates resolution
• Submits tx
• Persists hash
• Idempotent

Guidelines

• Minimum 90% test coverage on changed lines
• Input validation at the boundary; standardized error envelope
• Structured logging with correlation IDs
• Clear documentation and inline comments
• Timeframe: 96 hours

[AgilityB]

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

Hi Lead, kindly oblige me the opportunity to resolve this issue. I've got experience with similar tasks and I'm confident I can investigate, implement a clean solution, and thoroughly test it before submitting a PR.

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

[drips-wave]

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

🧑‍💻 @AgilityB: 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 🌊