Consecutive HTTP when posting thumbnails

[sentry-io]

This issue is to tell us that when we post thumbnails, we do them consecutively instead of in parallel. The latter would be better, probably using HTTPX and async?

Just an optimization, but potentially an educational one for the future when we are developing this kind of code.

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Sentry Issue: BIGCASES2-33

Offending Spans	http - POST https://bsky.social/xrpc/com.atprot...