Create a short summary of every email

[ChristophWurst]

Is your feature request related to a problem? Please describe.

As a Nextcloud Mail developer I want to show email summaries in the thread list: #10387

Describe the solution you'd like

Process incoming messages in the background. Store the result along with the email so this only happens once.

This feature will process user emails through a large language model. This needs an opt-in from the user.

Describe alternatives you've considered

N/a

Additional context

This is a prerequisite for #10387