feat(backend): Add request/response logging interceptor for admin endpoints

[Viky207]

Problem

Admin endpoints lack structured logging.

Task

Implement logging interceptor for admin/* routes.

Acceptance Criteria

• Captures method, path, status, latency, user agent
• Sensitive headers redacted
• Response bodies truncated to 1KB
• Correlation ID included
• Winston logger with admin.http namespace
• Sampling rate configurable via env