Replay loading fails due to concurrency limit

### Environment

SaaS (https://sentry.io/)

### Steps to Reproduce

Session Replays sometimes fail to load because the Sentry UI exceeds the replay endpoint’s concurrency limit (25), resulting in 492 errors and the message: "There was an error while fetching this Replay". I added the actual link to the internal Linear ticket.

### Expected Result

UI should respect concurrency limits and reliably load replays

### Actual Result

UI doesn't currently load certain replays

### Product Area

Replays

### Link

_No response_

### DSN

_No response_

### Version

_No response_

Start by reproducing the failure in the Sentry UI with Session Replays and observe requests to the replay endpoint, especially responses showing 492 errors. Trace how replay loads are requested and limited; done means the UI stays within the endpoint's concurrency limit of 25 and reliably loads replays without the reported error.

Written by the indexing model from the issue text.