Encode an export-compatible master for every clip that lacks one.

Covers the clips GET /project/{project_id}/export/preflight counts in renditions_needed_for_optimum_export — those with no export-compatible rendition at their own resolution — dispatching one downscale each, at that resolution. It is the highest rendition the sources allow, so afterwards every export reads the best master there is rather than a small proxy.

Assets with a downscale already running are left alone instead of being re-encoded, and an asset the caller cannot write to is skipped so its owner is not billed for an encode they did not ask for. Each encode is billed to the asset's owner as it runs, the same as POST /video/downscale/{id}; the amount reported here is the preflight's estimate of it.

An asset holds one proxy rendition at a time, so a new master replaces the smaller proxy that asset had.