Generates 400px-max thumbnails: PNG with alpha preserved when source has
transparency, JPEG (q=85, progressive) otherwise. Storage layout is
/images/thumbs/{sha[:3]}/{sha}.{jpg|png} so each prefix bucket holds at
most ~4k files (16^3 buckets across all possible SHAs).
Video path uses ffmpeg's first-frame snapshot at ~5% offset; we have a
1-second floor for short clips. ffmpeg call has a 60s timeout. Video
tests are deferred to FC-2e's integration suite where transcoding is
also wired up.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
