feat(backend): scaffold submissions module

[Xuccessor]

Summary

Contributors submit work against a bounty. Create the submissions module.

Acceptance Criteria

• POST /bounties/:id/submissions — create a submission (link, notes); requires JWT
• GET /bounties/:id/submissions — list submissions for a bounty (owner only)
• PATCH /bounties/:id/submissions/:subId/approve — approve and trigger payout; owner only
• PATCH /bounties/:id/submissions/:subId/reject — reject; owner only
• Approving a submission calls the Soroban contract release via the Stellar SDK
• Only one submission can be approved per bounty

[middalukawunti-lang]

hello, please can i work on this your project