feat(plugin-server): add ingestion warning when historical event timestamps too recent

[eli-r-ph]

Problem

Based on a support ticket discussion. Event capture routes events to our historical ingest pipeline if the incoming batch request is marked for historical_migration or if the event timestamps are older than 24 hours.

This is supposed to make the ingest events free of charge. However, the billing query will pick up the events regardless if the timestamp is in fact set to 24 hours ago or less.

After discussion with billing folks, we decided to update the migration documentation to more clearly call this out, and to consider adding an ingestion warning so users and PH admins have an audit trail when this happens.

This PR implements that warning.

Changes

• Adds an ingestion warning check to the plugin-server when an even comes through the historical ingestion pipeline with a timestamp less than 24 hours from "now" UTC.

Did you write or update any docs for this change?

Being handled separately ✅

How did you test this code?

Locally and in CI

[posthog-bot]

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[eli-r-ph]

After the corresponding docs update shipped, we haven't heard from any more confused users. Closing this out - can revisit if needed later 👍