feat: async investigate_fragment task; celery results backend

[jennifer-richards]

Moves the investigate_fragment() call out of the view and into a celery task. Updates the view to provide task polling support. Adds JS to start the task, poll for completion, and reload the results.

Adds the django-celery-results package so we can use the Django cache config to store task results. This replaces the unused rpc backend, which was not suitable for async results retrieval. Only stores results for the new task - others remain ignored - and only keeps them for 5 minutes after task completion.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.81%. Comparing base (4f53cad) to head (ba7f2d9).
Report is 14 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8428   +/-   ##
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  Coverage   88.81%   88.81%           
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  Files         312      312           
  Lines       40876    40930   +54     
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+ Hits        36304    36352   +48     
- Misses       4572     4578    +6     

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