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

frigate/record/maintainer.py

@@ -464,10 +464,12 @@ class RecordingMaintainer(threading.Thread):
                 self.drop_segment(cache_path)
                 return None
 
-        # if it doesn't overlap with an review item, go ahead and drop the segment
-        # if it ends more than the configured pre_capture for the camera
-        # BUT only if continuous/motion is NOT enabled (otherwise wait for processing)
-        elif highest is None:
+        # if it doesn't overlap with a review item, drop the segment once it
+        # ends more than event_pre_capture before the most recently processed
+        # frame. at this point we've already decided not to keep it for
+        # continuous/motion retention (either disabled or segment_stats said
+        # discard), so waiting longer just fills the cache.
+        else:
             camera_info = self.object_recordings_info[camera]
             most_recently_processed_frame_time = (
                 camera_info[-1][0] if len(camera_info) > 0 else 0