Record playback progress

Record episode playback position. Last-write-wins across devices (send the client's clock as client_timestamp). Marks the episode read once within 5% of its duration.

curl -X POST "https://example.com/v1/items/497f6eca-6276-4993-bfeb-53cbbbba6f08/playback-progress" \  -H "Content-Type: application/json" \  -d '{    "position_seconds": 0  }'

{  "playback_position": 0,  "last_played_at": "2019-08-24T14:15:22Z",  "is_read": false,  "completed": false}