Fix backend CI test cleanup

[Godsmiracle001]

Summary

• fix backend malformed JSON errors to return 400 with request IDs
• make migration dry-run config import-safe and testable
• replace stale Nest/rate-limiter tests with active Express middleware coverage
• update e2e deposit test payloads for token verification

Verification

• npm run test:backend
• npm run lint:backend
• npm run build:backend
• npm run verify:data-integrity
• git diff --check