fix: account for negative timestamps from rrweb

[daibhin]

We increasingly use the contents of rrweb events during ingestion so we need stricter validation...

We recently saw an issue caused by having received rrweb events from a client that had negative timestamps 🙃

This change ensures that timestamps we lift from rrweb events to create replay events are positive and result in a valid luxon datetime

[posthog-bot]

Hey @daibhin! 👋
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[pauldambra]

Thoughts on how we could level up the validation even more but not blocking since this is better already

[daibhin]

@pauldambra I didn't exactly know how to do it but another quick win would be to catch any invalid parameter and send it to Sentry. Debugging via logs / Kafka messages is always needle in a haystack kind of work