481b623f

By: Michael Lynch <git@mtlynch.io>

Collapse uploads.Processor into single-call ProcessImage/ProcessVideo

Replace the two-phase Processable-then-Process API with a single
ProcessImage(path, mediaType, files) / ProcessVideo(path, files) call each,
moving validation inside the process call instead of a separate
constructor step. This shrinks handlers' MediaProcessor interface from
four methods to two and simplifies publishImage/publishVideo into a
single call-then-branch-on-error shape.