[Backend] Backend KYC Provider Webhook Handler

[gabito1451]

Description:
Implement an authenticated webhook endpoint to receive updates from a KYC provider (e.g., Sumsub) and automatically submit the update to the Soroban Identity Contract.

Requirements and context:
Bridges off-chain KYC to on-chain registries.

Suggested execution:
Build the webhook route, verify HMAC signatures, and queue a Stellar transaction.

Guidelines:
High security required; failing signature validation must return 401 immediately.

[Abdullahi130]

@Abdullahi130 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I’m applying to build the KYC webhook handler for NovaFund backend. I have experience developing secure backend services, including webhook integrations, HMAC signature verification, and blockchain transaction workflows. I will implement an authenticated endpoint that validates provider signatures (e.g., Sumsub) and immediately rejects invalid requests with a 401 response. Verified events will be processed and queued for submission to the Soroban Identity Contract on Stellar. My approach focuses on security, reliability, and clean architecture, ensuring safe bridging of off-chain KYC data to on-chain identity records.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Abdullahi130 to this issue.