Dedicated LPR fixes (#17325)

* docs fps recommendation

* add detection_threshold to example

* send uncoverted yuv frame

* copy and convert

* ensure thumbnail is encoded as webp

* keep as jpeg

frigate/data_processing/common/license_plate/mixin.py

@@ -1279,10 +1279,9 @@ class LicensePlateProcessingMixin:
             # save the best snapshot
             logger.debug(f"Writing snapshot for {id}, {top_plate}, {current_time}")
             frame_bgr = cv2.cvtColor(frame, cv2.COLOR_YUV2BGR_I420)
-            _, buffer = cv2.imencode(".jpg", frame_bgr)
             self.sub_label_publisher.publish(
                 EventMetadataTypeEnum.save_lpr_snapshot,
-                (base64.b64encode(buffer).decode("ASCII"), id, camera),
+                (base64.b64encode(frame_bgr).decode("ASCII"), id, camera),
             )
 
         self.detected_license_plates[id] = {