Unit tests for isPast and rsvpId #99

What

isPast and rsvpId in lib/events.ts decide whether an event is over, and build RSVP document ids.

There are no tests for it yet.

Cover at least

• an event whose endsAt is before now is past
• with no endsAt, it falls back to startsAt
• an event ending exactly at now is not past
• pass now explicitly, never rely on the real clock
• rsvpId is deterministic, and differs for different users or events

How

• Create lib/events.test.ts next to the code. Follow the style of lib/github-auth.test.ts.
• Run npm test. Then break the code on purpose (flip a condition) and watch at least one of your tests fail. Paste that failure output in the PR description, then undo the break.
• Test behaviour through the exported function only. Don't change the source file unless you find a bug, and if you do, stop and comment on this issue first.

Size: ~30–60 lines of tests, 0 lines of source.

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