Add "Bandwidth Saving" mode which only transmits video when participants are unmuted.

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Feature request

Which Nextcloud Version are you currently using: (see administration page)
Nextcloud Hub 10 (31.0.7)
Is your feature request related to a problem? Please describe.
In low-bandwidth environments, calls may become more unreliable, being able to set calls to only send video when a user is unmuted could drastically reduce bandwidth demand and increase reliability, especially without the high performance backend.

Describe the solution you'd like
In the ongoing calls settings, or when a call is being organised, there should be a checkbox to enable "low bandwidth mode", this could also throttle video quality if needed.

Describe alternatives you've considered
Just turning of cameras, and having users reenable their camera and microphone every time they want to speak, however this is slow as camera reauthorisation is required, or camera startup is slow.

Additional context
n/a

Start by tracing the ongoing-call settings and participant mute/camera behavior described in the request; no file or test entry point is named. Done means an agreed low-bandwidth mode transmits video only while participants are unmuted, with any quality-throttling behavior defined and verified.