fix(indexer): remove duplicate soroban indexer service (#801)

[Banx17]

[npm run lint ✅ completed (existing repo warnings only)
npm run typecheck ✅ passed
npm run build ✅ passed]

Overview

This PR resolves a race condition where two indexers (soroban-indexer.service and soroban-event-worker) were running concurrently, leading to a double-counting of withdrawnAmount for the same events. The legacy service has been removed so that only the SorobanEventWorker polls the contract. Additionally, the WITHDRAWN handler has been made idempotent to guarantee that the same event cannot increment the balance more than once.

Related Issue

Closes #801

Changes

⚙️ Indexer Consolidation

• [DELETE] backend/src/services/soroban-indexer.service.ts
• Removed the legacy indexer service to ensure only a single indexer processes events.
• [MODIFY] backend/src/index.ts
• Removed the sorobanIndexerService.start() call and related graceful shutdown logic.

🛡️ Idempotency Improvements

• [MODIFY] backend/src/workers/soroban-event-worker.ts
• Updated handleTokensWithdrawn to check for the existing StreamEvent record before applying the balance increment, preventing double-counting if the same event is processed twice.

🧪 Tests

• [DELETE] backend/tests/soroban-indexer.test.ts
• Removed obsolete tests for the legacy service.
• [ADD] backend/tests/single-indexer.regression.test.ts
• Added a regression test asserting only one indexer instance polls the contract.
• Added a test to verify withdrawnAmount is not double-incremented when the same WITHDRAWN event is observed.

Verification Results

Acceptance Criteria	Status
Remove sorobanIndexerService.start()/stop() from index.ts	✅
Delete or disable services/soroban-indexer.service.ts	✅
Add a regression test asserting only one indexer instance polls the contract	✅
Verify withdrawnAmount is not double-incremented when the same WITHDRAWN event is observed	✅
Backend tests pass successfully	✅