feat(chapter-05): add structured logs with request IDs

Chapter: 05 - Observability.

Plain-English map:
Add Pino JSON logging and attach a request ID to each request so related log
lines can be traced together in production.

Why it matters:
Plain text logs are hard to search once many users are active. Structured logs
let operators answer practical questions like "what happened during this one
failed upload?"

Principle:
Production logs should be queryable, consistent, and tied to request context.

Precedent borrowed:
Upstream PR #156 and common production logging practice in hosted services.

Upstream base: willchen96/mike@d39f580.
Original local commit: 6f59af5.