Add slack secret to celery

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.20%. Comparing base (65f5830) to head (e5aacb9). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #680 +/- ## ======================================= Coverage 94.20% 94.20% ======================================= Files 80 80 Lines 3279 3279 ======================================= Hits 3089 3089 Misses 190 190 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.