fix(alerts): respect latest data for hourly trends alerts

[gesh]

Problem

Hourly trends alerts are not calculated properly.
For the cache key, we use relative dates (like -2h).
We cache the trends value for the past hour using the relative date. Then, after 1 hour, for the next check, we use the cached value, which is not valid now.
Making the less expensive alerts calculations to ignore the caching, so they use the latest data, and prevent spamming alerts.

https://posthoghelp.zendesk.com/agent/tickets/39356

How did you test this code?

Manually + Added some tests

[greptile-apps]

Additional Comments (1)

1. posthog/tasks/alerts/trends.py, line 305-309 (link)

   syntax: duplicate call to _pick_series_result - the second call (lines 308-309) does nothing since the result isn't used

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[anirudhpillai]

Looks great, thanks for adding the test!
Hourly insight results are cached long enough for this issue to come up, for other intervals you're right, this shouldn't be an issue