Purging completed tasks automatically / in bulk

[4oo4]

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Is your feature request related to a problem? Please describe.
Using the tasks app it's very easy to accumulate many completed tasks. When using external tasks applications (GNOME Tasks/Endeavour, Outlook CalDav Sync) , all of these completed tasks get synced and it causes performance issues and in some cases can make external CalDav task apps almost unusable.

nextcloud/tasks#2309

nextcloud/tasks#2125

https://help.nextcloud.com/t/mass-editing-deleting-tasks/26070

Describe the solution you'd like
A mechanism in server config to allow for deleting completed tasks after an interval, that could be ran with Nextcloud's other scheduled cron jobs.

Describe alternatives you've considered
I can seem to go into each individual completed task and delete it, but I don't seem to have a mechanism to delete them batches. I have over 1000 completed tasks so this could easily take hours to delete the in the web UI, and just attempting to load them takes several minutes and freezes my browser tab. I could also delete them directly from the database but this would raise the possibility of breakage/data loss.

Additional context
Per the developer of the tasks app, this is something that would need to be done server-side (while the tasks app is client-side), so I'm creating the issue here instead of the tasks app.

nextcloud/tasks#2125

[meonkeys]

Here's one workaround if you have GNOME Todo/Endeavor:

1. choose a nextcloud task list from those listed (heads up: mine shows both task lists and calendars)
2. an ellipsis ... (3 dots, formerly hamburger) menu will appear in the window title bar somewhere (upper right for me), click it
3. click "Clear completed tasks"

If you have 1,000+ tasks, be very patient while 1,000+ HTTP DELETE requests are sent to the server. It does the job, though. 🧹