Validate notification expiration timestamps on registration

[michaelvic123]

closes #96

What

Prevents registration of notifications with invalid or already-expired executeAt timestamps.

Changes

• notification-api.ts — split the single vague guard into two checks:
  • Rejects non-Date or Invalid Date objects: executeAt must be a valid date
  • Rejects timestamps at or before now: executeAt must be a future timestamp — the provided date has already expired
• events-server.ts — validates the raw executeAt string before constructing a Date, returning 400 early instead of passing an Invalid Date silently downstream
• Tests — replaced the single past-date test with four boundary cases:
  • expired timestamp (60s ago)
  • timestamp equal to now (boundary)
  • new Date('not-a-date') (invalid Date object)
  • 1 second in the future (should pass)

Testing

npm test -- --testPathPattern=notification-scheduler

All 30 tests pass.