feat: Implement and test POST /api/kyc/submit endpoint

[Olowodarey]

This PR implements the user-facing KYC submission endpoint (POST /api/kyc/submit) and introduces a comprehensive integration test suite covering:

• Authentication enforcement
• Idempotent behavior
• Response shape validation

While the route handler and KycService::submit_kyc method were already scaffolded, they lacked test coverage. This PR closes that gap and ensures the endpoint is production-ready and verifiable.

---

What Was Done

1️⃣ Endpoint Verification

Confirmed that:

• POST /api/kyc/submit is correctly wired in app.rs
• The handler uses the AuthenticatedUser extractor
• The request is delegated to KycService::submit_kyc
• The service performs an upsert on the kyc_status table:

INSERT ... ON CONFLICT DO UPDATE

• The record is inserted (or updated) with:

  status = 'pending'
  

This guarantees idempotent behavior for repeated submissions.

---

2️⃣ Integration Test Suite

Added 5 integration tests in:

tests/kyc_submit_tests.rs

Key characteristics:

• Uses the real Axum router via create_app
• No mocked HTTP handlers
• Tests exercise the full request → handler → service → database flow

---

Test Coverage Includes

• ✅ Rejects unauthenticated requests
• ✅ Accepts authenticated requests
• ✅ Inserts a new kyc_status row with pending status
• ✅ Ensures idempotency (repeat submissions do not create duplicates)
• ✅ Validates response structure and status codes

---

Technical Notes

• Router created via create_app
• Uses AuthenticatedUser extractor for auth enforcement
• Ensures database upsert logic behaves as intended
• Improves overall API reliability and confidence before frontend integration

---

Checklist

• Endpoint wiring verified
• Integration tests added
• Authentication behavior validated
• Idempotency confirmed
• Response shape validated
• All tests passing locally

closes #126

[ONEONUORA]

Nice implementation @Olowodarey