add snapshot time to event data

docs/docs/usage/mqtt.md

@@ -37,6 +37,7 @@ Message published for each changed event. The first message is published when th
     "id": "1607123955.475377-mxklsc",
     "camera": "front_door",
     "frame_time": 1607123961.837752,
+    "snapshot_time": 1607123961.837752,
     "label": "person",
     "top_score": 0.958984375,
     "false_positive": false,
@@ -54,6 +55,7 @@ Message published for each changed event. The first message is published when th
     "id": "1607123955.475377-mxklsc",
     "camera": "front_door",
     "frame_time": 1607123962.082975,
+    "snapshot_time": 1607123961.837752,
     "label": "person",
     "top_score": 0.958984375,
     "false_positive": false,

frigate/object_processing.py

@@ -153,10 +153,16 @@ class TrackedObject:
         return significant_update
 
     def to_dict(self, include_thumbnail: bool = False):
+        snapshot_time = (
+            self.thumbnail_data["frame_time"]
+            if not self.thumbnail_data is None
+            else 0.0
+        )
         event = {
             "id": self.obj_data["id"],
             "camera": self.camera,
             "frame_time": self.obj_data["frame_time"],
+            "snapshot_time": snapshot_time,
             "label": self.obj_data["label"],
             "top_score": self.top_score,
             "false_positive": self.false_positive,