[Backend] Wire commitPrediction to real blockchain call

[GoSTEAN]

Description

prediction.service.ts line 96 uses const txHash = 'mock-tx-hash-' + Date.now(). Must call Market.commit_prediction() on Soroban.

File: backend/src/services/prediction.service.ts

Acceptance Criteria

• Approve USDC spending → invoke commit_prediction → wait for confirmation
• Store real tx hash in database
• Handle blockchain errors with DB rollback
• Integration test

Priority

🔴 P0 — Critical