Stop browsers that weren't accessed in a long time.

[SvenMarcus]

I had a bit of time on my hand, so I implemented an initial idea for stopping browsers that weren't accessed in a long time to free up used ports.

Whenever a user opens a workspace, the server updates the access time of the corresponding background process.
The clean up is happening during our lifespan() function as an infinitely running async background task.

In order for this to work I added the methods update_access_time(browser) and browsers_accessed_before(time) to the repositories.
processtimeout.expiration_loop() is the background task called in the lifespan.

Note that in order to be able to test this functionality, I had to create a special lifetime for our tests, as the continuous loop would have prevented our tests from ever completing. You can find it in tests/ocrdmonitor/fixures/lifespan.py
It uses the same tasks as the production lifetime (found in ocrdmonitor/main.py), but with different arguments for expiration_loop to only loop once.

The timeout for browsers is configured in init.sh with the new environment variable OCRD_BROWSER__TIMEOUT.
Its value is a duration in ISO 8601 format as that is supported out of the box by pydantic (https://docs.pydantic.dev/latest/api/standard_library_types/#datetimetimedelta)