⚡ Switch from prefork to threads for Celery concurrency

[stevenbal]

Because Open Notificaties is primarily I/O bound, the Celery workers could spend a decent amount of time waiting, and in this scenario it is more efficient to use threads because they have a lower memory footprint than processes

TODO:

• Push custom docker tag and let DH loadtest with this
• adjust default concurrency numbers in charts?
• Should we apply the same for apps that send to open notificaties?
• reuse session in tasks

Changes

• Switch from prefork to threads for Celery concurrency
• Disable DB connection pooling and increasy Celery concurrency for docker-compose
• Update webhook demo setup to use fastAPI

Checklist

Check off the items that are completed or not relevant.

• Experimental features/changes

  • Any experimental features added in this PR are backwards compatible
  • Any experimental features added in this PR are documented in the docs/api/experimental.rst page

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.68%. Comparing base (8bef94b) to head (957c662).
⚠️ Report is 27 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #375   +/-   ##
=======================================
  Coverage   93.68%   93.68%           
=======================================
  Files         124      124           
  Lines        4231     4231           
  Branches      666      666           
=======================================
  Hits         3964     3964           
  Misses        205      205           
  Partials       62       62           

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[stevenbal]

Closing this in favor of #364 since that also introduces the same change