Ability to record long VMs by recording in the background

I find it pretty common to want to send a long VM (e.g. of a voice memo to myself, or recording some music, etc) and the very last thing I want is for the screen to lock and suddenly truncate the VM - especially if i don't notice it happening.

We should keep recording the VM in the background (which was the case until very recently anyway) - *BUT* we should force the user to very explicitly hit send once they next fg the app, to prevent pocket-dialling incidents.

Start by tracing the current VM recording lifecycle and how screen locking or app backgrounding affects it. Check the existing send flow and background recording behavior; done means a long recording survives locking and requires an explicit send action when the app returns to the foreground.

Written by the indexing model from the issue text.