Reject some requests that may conflict with project stats refresh

What does this MR do and why?

This is continuation from !86686 (diffs, comment 945808728). As suggested, we are splitting the work into 3 separate MRs. This is MR 2 of 3.

Part of the work for #360375 (closed).

In this MR, we add the 409 conflict guards in certain endpoints that are known to cause artifact deletion. If a project is undergoing stats refresh, we will reject the request to avoid artifact deletion that may mess up the ongoing stats recalculation.

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