Fixes (#24631)

* fix classification wizard finding no sample images on large databases

The wizard grouped tracked objects by camera and 6 hour block, oldest first, then kept the first 100. With more than 100 groups that was always the oldest objects, which often have no snapshot or thumbnail left on disk, so no examples were generated. Shuffle the selection before truncating, and keep extracting from the remaining tracked objects until 100 usable images are found.

* add notice and system ui message when a camera isn't using go2rtc

* add docs note about privacy masks

* match index-less device strings to hardware units in the detection models picker

A config with `edgetpu:usb` was reported as hardware that wasn't found, because the picker compared device strings exactly and the probe reports the unit as `edgetpu:usb:0`. A device without an index now resolves to the first unit of that kind, so the hardware dropdown, the unit checkboxes, and the model summary all recognize it.

* don't treat unknown audio as an audio-bearing stream

A recording row with NULL has_audio (ffprobe failed and the cv2 fallback can't report audio) marked the whole stream as audio-bearing, so every confirmed video-only row on it was dropped as a glitch. A stream now counts as audio-bearing only when a row is known to carry audio.

* stage main+sub exports on disk instead of /tmp/cache

A main+sub export stages a full copy of itself before the final file is written. That copy went to /tmp/cache, so a long export outgrew the tmpfs and failed with no space left on device. Staged runs are now written to the exports directory, and startup removes any left behind by a killed export.

* address review feedback

- stage main+sub export runs in a staging subfolder of the exports directory, so media sync can't delete them mid-export and startup cleanup can't remove a finished export with a matching name or fail on an unremovable file
- run object classification example collection off the event loop
- prefer an exact hardware unit match over an index-less one, so an AMD GPU's "onnx" no longer selects the NVIDIA entry
- add tests for the classification fallback and index-less hardware matching

* serialize example collection and clean up exports when staging dir creation fails

- overlapping object example requests could delete each other's images in the shared temp and train directories, so collection now runs under a lock
- a failed makedirs for the staging directory raised past the failed-export cleanup and left the export spinning until restart, so it now fails the run through the normal cleanup path

* revert

docs/docs/troubleshooting/faqs.md

@@ -53,6 +53,14 @@ go2rtc:
 
 Point the camera's inputs at the restream as described in the [restream docs](/configuration/restream.md), and swap `detect -> width` and `detect -> height` to match the rotated resolution.
 
+### Can I add a privacy mask to hide part of my camera's view?
+
+Frigate does not have privacy masks. [Motion masks and object filter masks](../configuration/masks.md) only affect detection, they don't hide anything in live view, recordings, or snapshots.
+
+Privacy masks are best configured in the camera's firmware settings so the area is blacked out before the video ever leaves the camera and no extra processing is needed. Check there first.
+
+If your camera does not support privacy masks, there is no efficient alternative. Frigate copies the camera's video into recordings and live view without decoding it, so part of the image can't be hidden without transcoding (re-encoding) the stream. This can be done with a custom ffmpeg filter in go2rtc, but it is not recommended. Every masked camera needs a continuous re-encode, which significantly increases CPU usage, especially for high resolution streams.
+
 ### My mjpeg stream or snapshots look green and crazy
 
 This almost always means that the width/height defined for your camera are not correct. Double check the resolution with VLC or another player. Also make sure you don't have the width and height values backwards.