fix(subscriptions): cap png export height to prevent OOM

[andyzzhao]

Problem

There are two issues with exporting images:

1. There are some large insights (tables with thousands of rows or giant rows) that eat up memory when we try to screenshot them.

2. In addition, the thread running the screenshot sometimes continues even after the top level coroutine times out, resulting in 100% memory usage after the temporal workflow has completed. Temporal is able to retry activities when we the pods get OOM killed, but if there are too many OOMs in a single workflow, the retries get exhausted.

related convo: https://posthog.slack.com/archives/C09BDQLM8KA/p1761925099100499

Changes

Address, the first issue directly by limiting the number of pixels we screenshot. This should indirectly help the second issue, as the screenshot shouldn't run as long with fewer pixels to capture.

How did you test this code?

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Locally. Verified that a png export is shorter with the new setting.

Changelog: (features only) Is this feature complete?

[andyzzhao]

The caveat with this approach is that chrome still has to load the entire table, but at least it doesn't have to screenshot the entire table.

[greptile-apps]

Additional Comments (1)

1. ee/tasks/subscriptions/subscription_utils.py, line 122 (link)

   logic: Celery task path doesn't pass height limit, so generate_assets() (used by some subscriptions and Slack sharing) won't get OOM protection that generate_assets_async() has

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