feat(backend): add request ID middleware for distributed tracing

[GRACENOBLE]

Problem

No correlation ID is generated or propagated. When a request fails it is impossible to correlate the client error with a specific backend log line, especially once Sentry and structured logs are both in play.

Expected behaviour

• Every request gets a unique ID (UUID v4 or similar)
• The ID is read from X-Request-ID if the caller provides one, otherwise generated
• The ID is set on the Gin context and included in all log lines for that request
• The ID is returned in the X-Request-ID response header

Acceptance criteria

• New middleware in internal/transport/middleware/request_id.go
• Middleware registered before the logger in routes.go
• Logger middleware reads the ID from context and includes it in every log entry
• Documented in backend/docs/

[GRACENOBLE]

Implemented and merged via PR #65.