Miscellaneous fixes (#22913)

* add log when probing detect stream on startup

when users don't explicitly set detect.width and detect.height, we probe for them. sometimes the probe hangs (camera doesn't support UDP, like some Reolinks), so this log message will make that clearer

* add faq about probing detect stream

* fix stuck activity ring when tracked object transitions to stationary

* drop cache segments past retain cutoff regardless of retention mode

* add maintainer test