Add cancelReminder and getPendingReminders to invoiceReminderScheduler

[Kingsman-99]

Description

src/invoiceReminderScheduler.ts schedules reminders for invoice deadlines but provides no way to cancel a previously scheduled reminder. If an invoice is paid early or the deadline changes, the reminder fires anyway with no cancellation path.

Acceptance Criteria

• scheduleReminder returns a reminderId: string (opaque unique identifier)
• cancelReminder(reminderId: string): boolean exported — cancels the reminder and returns true; returns false if the ID is unknown or already fired
• getPendingReminders(): Array<{ reminderId: string; invoiceId: string; remindAt: number }> exported — returns all not-yet-fired, not-cancelled reminders
• Cancelled reminders do not fire even if their scheduled time passes
• clearAllReminders(): void exported for test teardown
• All exported from src/index.ts
• Unit tests: cancel before fire prevents callback, cancel unknown ID returns false, cancel already-fired returns false, getPendingReminders excludes cancelled

Context

• Target files: src/invoiceReminderScheduler.ts, src/index.ts
• Use fake timers in tests (vi.useFakeTimers) to control time without real delays

[bobo-24]

Hi! I’m a full-stack developer with a strong blockchain background, and I’m confident I can resolve this issue quickly and effectively. I’ll deliver a clean, reliable solution with immediate attention to detail, ensuring it’s completed as soon as possible. Please assign it to me—I’m ready to get started right away.

[afeezorobsco-cyber]

I would like to work on this issue

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #616)
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