Allow blank cron schedule (manual process only) or alternatively don’t process on startup

[rjbutler]

Is your feature request related to a problem? Please describe.
While testing prompts, I’d prefer to only manually process. Paperless-ai seems to immediately process on startup, so even setting a cron schedule that is very long (weekly, monthly, etc), documents still get processed when I restart the docker container.

Describe the solution you'd like
There are several options: An explicit setting for manual processing only, or allowing a blank cron schedule that means the same, or only process based on the cron schedule not on startup.

[mamema]

+1, i also always wait for the app to stop, because until then no config change is possible, because the app is not responding. For example at the moment i'm trying to login since 5 minutes, logs not showing anything problematic, it just isn't responding.
Until now i flagged my docs in low figure batches just to stop paperless-ai to do "too much" and getting unresponsive.