Executes a /keyframes job: writes the base64 MP4 to a temp file, probes duration, extracts
up to MaxFrames (<= 24, default 16) scene-change + uniform-sample keyframes via
backend.ExtractKeyframes, and optionally builds contact sheets when BuildContactSheet is true.
Executes a
/keyframesjob: writes the base64 MP4 to a temp file, probes duration, extracts up toMaxFrames(<= 24, default16) scene-change + uniform-sample keyframes viabackend.ExtractKeyframes, and optionally builds contact sheets whenBuildContactSheetis true.