Refunds: Refund status tracking via indexer

[Akanimoh12]

Summary

Implement “Refund status tracking via indexer”.

Part of building the real-time off-chain backend for Stellar Tipz. All backend work happens on the test-implement-drips branch — branch off it and open your PR against it (not main).

Tasks

• Implement per module conventions.
• Add tests.

Acceptance criteria

• Authorized; tests + typecheck + lint pass.

Files / location

• backend/src/modules/refunds/

Getting started

1. git checkout test-implement-drips && git pull
2. cd backend && npm install
3. docker compose -f backend/docker-compose.yml up -d (Postgres + Redis)
4. Read backend/README.md and backend/docs/BACKEND_CONTRIBUTING.md.
5. Follow the module conventions; run npm run typecheck && npm run lint && npm run test before opening your PR.

Definition of Done for every issue is in BACKEND_CONTRIBUTING.md.