Add position_seconds (+progress_updated_at) to video_states so watch progress
survives across devices and can drive a feed filter. New POST
/api/videos/{id}/progress checkpoints the player position (clearing trivially
-early and near-finished positions). Feed serialize exposes position_seconds and
a show=in_progress filter lists started-but-unfinished videos. Un-marking
'watched' now keeps a stored position instead of deleting the row.
