Build scheduled release countdown utility

[Kingsman-99]

Labels: complexity: high

Add getScheduledReleaseCountdown(invoice) that computes the time remaining until a scheduled release fires. Returns a structured object with days, hours, minutes, seconds, and whether the release is overdue.

Acceptance criteria:

• Returns { total_seconds: number, days, hours, minutes, seconds, overdue: boolean }
• Returns null when invoice has no scheduled_release_at
• Pure function, no RPC calls needed
• Exported with full TypeScript types

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