feat: add prediction cancel endpoint with refund (#180)

[CyberXpert607]

Summary

Implements #180: Add /api/predictions/:id/cancel endpoint with refund for unresolved predictions.

Changes Made

1. Cancel Endpoint (src/routes/predictions/cancel.ts)

• ✅ POST /api/predictions/:id/cancel
• ✅ Validates prediction ownership and market status
• ✅ Refunds stake to user's balance (immediate credit)
• ✅ Updates prediction status to 'cancelled'
• ✅ Structured logging with correlation ID

2. Route Registration (src/routes/predictions.ts)

• ✅ Imported and registered cancelRouter
• ✅ Authentication applied via requireAuth middleware

3. Tests (tests/routes/predictions/cancel.test.ts)

• ✅ Success scenario (cancel + refund)
• ✅ Non-existent prediction → 404
• ✅ Market settled → 400
• ✅ Already cancelled → 400
• ✅ Authentication required → 401
• ✅ Coverage ≥90%

4. Error Handling

• ✅ 400: Market settled or cancelled
• ✅ 400: Prediction already cancelled
• ✅ 404: Prediction not found
• ✅ 401: Unauthorized
• ✅ 500: Internal server error

Related Issue

Closes #180

Checklist

• Cancel endpoint implemented
• Refund logic in place
• Input validation at boundary
• Structured logging with correlation ID
• Tests added and passing (≥90% coverage)
• Route registered correctly
• Secure (authentication required)

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