Build invoice deadline engine with multi-timezone countdown, business-hours awareness, and expiry callbacks

[Kingsman-99]

Description

Build a comprehensive deadline management module that provides human-readable countdowns in multiple timezones, skips non-business hours when computing "effective" time remaining, fires registered expiry callbacks when a deadline passes, and integrates with the existing watcher.ts polling system.

Acceptance Criteria

• Create src/deadlineEngine.ts exporting DeadlineEngine class
• getCountdown(deadline: number, options?: CountdownOptions): CountdownResult — returns { expired: boolean, display: string, secondsRemaining: number, localDisplay: string }
• CountdownOptions supports: timezone?: string (IANA, e.g. "America/New_York"), businessHours?: { start: number, end: number, days: number[] }
• When businessHours is set, secondsRemaining counts only business-hours seconds remaining (e.g. a deadline 10 hours away at 5pm Friday = only next Monday morning hours count)
• registerExpiryCallback(deadline: number, invoiceId: string, cb: (invoiceId: string) => void): () => void — returns unsubscribe function; callback fires once when deadline passes
• Callbacks are managed via setInterval internally; the interval is shared (one per DeadlineEngine instance), not one per callback
• destroy() clears all callbacks and the interval
• Exported from src/index.ts
• Tests: business-hours calculation across weekend, callback fires after mock time advance, unsubscribe prevents callback, multiple callbacks on same deadline all fire, destroy clears all

Context

• isExpired in src/utils.ts is the existing baseline
• src/watcher.ts uses polling — DeadlineEngine should be usable standalone or alongside it
• Business hours must handle DST correctly using Intl.DateTimeFormat — do not use moment.js or external date libraries