Save snapshot from different stream than detection stream.

**Describe what you are trying to accomplish and why in non technical terms**
I want to be able to run detection on a lower, more efficient stream, but then use a higher resolution stream to pass the MQTT image and for saving the snapshot. This will let me stay efficient on detection, but get high-res screens via MQTT to other apps like HA, DoubleTake, and custom scripts.

**Describe the solution you'd like**
A "snapshots" role in the camera section would allow separation between the detection stream (which currently is used for snapshots) and the stream used to capture snapshots. Further, an "mqtt_snapshot" role could be specified for the images passed over MQTT on detection events.

**Describe alternatives you've considered**
Running the detection on the high res stream, but that's inefficient with many cameras.

Start by tracing how the camera detection stream currently supplies snapshots and MQTT event images. Review the camera configuration and stream-selection paths, then determine how separate snapshot and mqtt_snapshot roles would fit. Done means each role can select its own stream without breaking existing detection, snapshot, or MQTT behavior.

Written by the indexing model from the issue text.