Integration deletion is very slow on uninstall

## Summary

Deleting a Sentry app integration (clicking "Uninstall" on a configuration) is very slow — it does not complete instantly as expected. The operation takes a noticeably long time, making the UI feel unresponsive.

## Symptom

- Navigate to an installed integration's configuration page
- Click "Uninstall" / delete the integration
- Deletion is slow; the user waits several seconds (or longer) before the operation completes

## Expected behavior

Deletion should be near-instant or at minimum provide responsive feedback. The API call triggering deletion is likely synchronous and blocking on work that could be deferred (e.g. cascading deletes, async cleanup tasks).

## Root cause hypotheses

- **High confidence:** Deletion endpoint performs synchronous cascading cleanup (e.g. uninstalling webhooks, revoking tokens, cleaning up installations) in the request/response cycle rather than deferring to a background task.
- **Medium confidence:** The `SentryApp` or `SentryAppInstallation` deletion path may be doing database-level cascades or firing external API calls inline.

## Unknowns

- Exact endpoint and view handling the delete request
- Whether there is any task deferral currently in place
- Scale of the slowdown (seconds vs. tens of seconds; consistent or variable by integration)

Action taken on behalf of David Cramer.

No file, test, or endpoint is named in the issue. Trace the integration uninstall request and its SentryApp or SentryAppInstallation deletion path, identify synchronous cleanup or external calls, and verify that deletion completes promptly without blocking the request.

Written by the indexing model from the issue text.