Add structured logging example using event callbacks

[TheCryptoDonkey]

toll-booth emits onRequest, onChallenge, and onPayment callbacks that give operators visibility into the payment flow. There is no example showing how to wire these into a structured logging setup (JSON lines, pino, etc.). A working example would help operators monitor their deployments in production.

Scope

• Create examples/structured-logging/ with a minimal Express server
• Wire onRequest, onChallenge, and onPayment callbacks to produce JSON log lines to stdout
• Include fields like timestamp, client IP, route, amount, payment hash, and credit balance
• Add a README.md explaining the callback signatures and when each fires

Helpful context

• src/types.ts lines 347-349 - the three event callback signatures (onPayment, onRequest, onChallenge)
• src/stats.ts - the built-in StatsCollector that uses these same callbacks internally
• src/booth.ts lines 128-132 - how the Booth constructor wires user callbacks alongside stats collection
• Estimated effort: 1-2 hours