Processing timeout

[devmount]

Is your feature request related to a problem? Please describe.
We still have users issuing never ending processing on the stats page.

Describe the solution you'd like

• We should add a reasonable processing timeout, like e.g. 2 minutes for one folder (which is plenty, even on low end CPUs and folders with > 10k messages).
• If we ran into a timeout, that should be hinted to the user after processing finished

Maybe that solves most of the never ending processing issues and gives more hints why that happens.

Additional context
Related to #445