Add the video direction to a Song DNA song

For a finished song analysed with mode: "dna": builds the story and the beat-synced shot plan from the existing analysis (no new upload) and turns the song into a full song. Costs 15 credits (never a free song), refunded automatically if it fails. The song is processing for about 30 seconds, then done again; a webhook_url set on the song gets a second song.done event with its own id and data.direction_added: true.