Implement submission review endpoints (approve/reject)

[bishopBethel]

Build POST /submissions/:id/approve and /submissions/:id/reject for bounty creators. Approve triggers payment flow. Reject requires rejection_reason and transitions submission status.

[devasign-app]

💵 13.00 USDC Bounty

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[chengyixu]

I'd like to work on this.

Plan:

1. Implement POST /submissions/:id/approve -- validate submission exists and belongs to the bounty creator, update submission status to 'approved', trigger payment flow
2. Implement POST /submissions/:id/reject -- validate rejection_reason field, update submission status to 'rejected', transition state properly
3. Add input validation, proper error handling and status codes
4. Add tests for both endpoints including edge cases (already approved/rejected, unauthorized access)

Timeline: PR within 24 hours.

[bishopBethel]

@chengyixu kindly apply above. check devasign-bot comment

[zhaog100]

/attempt

[zhaog100]

/attempt