add logging

[DanielEmmanuel1]

feat: add structured logging to stellar wallet service

Closes #142
This PR introduces structured, centralized logging for the Stellar Wallet service to improve debugging, monitoring, and auditing.

• Added winston and configured JSON logging with timestamps to console and logs/app.log
• Implemented loggerMiddleware to log all requests/responses (method, URL, status, duration, user_id)
• Replaced ad-hoc console errors with structured logs across critical paths:
  • src/stellar/sign.ts (transaction signing)
  • src/routes/kyc.ts (KYC submission)
  • src/routes/kyc-verify.ts (Soroban registration)
  • src/routes/wallet.ts (account creation and funding)
• Applied middleware early in src/index.ts
• Added LOG_LEVEL env var (default info) and ignored logs/ in git
• Added unit test tests/middlewares/logger.test.ts with mocked Winston
  Notes
• Sensitive data is not logged; user_id included when available
• CI: lint and tests pass locally on feat/logging

[salazarsebas]

LGTM