[Backend] Wire revealPrediction to real blockchain call

## Description

`prediction.service.ts` line 179 uses `const revealTxHash = 'mock-reveal-tx-' + Date.now()`.

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

## Acceptance Criteria

- [ ] Call Market.reveal_prediction(market_id, prediction, salt)
- [ ] Verify on-chain confirmation
- [ ] Store real reveal tx hash
- [ ] Integration test

## Priority
🔴 **P0 — Critical**