Implement KYC status tracking in merchant model`

[Nemenwq]

Dependencies: Issue #27, Issue #28.

Context:
Compliance requires tracking merchant KYC state.

Scope:

• In scope: Schema fields + submit endpoint.
• Out of scope: Integration with KYC vendor.

Tasks:

• Add merchant fields:
  • kyc_status (e.g. NOT_SUBMITTED, PENDING, APPROVED, REJECTED)
  • kyc_submitted_at, kyc_reviewed_at, kyc_rejection_reason
• Add endpoints (dashboard-authenticated):
  • POST /kyc/submit (marks status PENDING)
  • GET /kyc/status

Implementation Notes:

• Keep the payload minimal for now (no document uploads in this issue).
• Add audit log entry when status changes (ties into Issue Implement audit log table and service` #52).

Files: backend/api_gateway/src/merchants/