`get_duration_seconds` now returns `Option<f64>` and falls back from
`format=duration` to `stream=duration`. Empty stdout no longer
parse-panics with "cannot parse float from empty string", which was
poisoning the preview-clip row with status=failed and re-queueing every
full scan (notably for GoPro LRV files). `generate_preview_clip` handles
the unknown-duration case by transcoding the whole file (capped at 10s).
`generate_video_thumbnail` seeks to ~50% of the probed duration instead
of a hardcoded `-ss 3`, with a first-frame fallback when the probe
returns nothing. Fixes the loop where short Snapchat clips (<3s) got
"missing thumbnail" logged on every scan because ffmpeg exited 0
without writing a frame, and never wrote the .unsupported sentinel
either.
Adds unit tests for `parse_ffprobe_duration` covering the empty-output,
N/A, multi-line, non-positive, and non-finite cases.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
