Story images that aren't 9:16 were clipped by Instagram. They are now
fitted onto a 1080x1920 canvas — the image is contained and a blurred,
darkened copy of itself fills the letterbox gaps, so nothing is cropped
and the background color adapts to the image.
The fit happens at publish time (the hosted copy lives in social-crops/),
and the post editor preview now renders the same blurred-background fit
for stories, so the user sees exactly what will publish. The aspect-ratio
warning is suppressed for story images since they're auto-fitted.
Instagram only — Facebook stories are video-only in our flow.
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