Oban job for background processing of uploaded files.

This job handles:

• Generating file variants (thumbnails, resizes)
• Extracting metadata (dimensions, duration)
• Updating file status

Unique per file_uuid while still pending or running, because the job regenerates every variant of a file rather than one: a gallery page whose thumbnails do not exist yet asks FileController for a dozen variants at once, and without this each request would enqueue another full run of the same work. The key is the file, not the variant, for the same reason — the args carry no variant at all.

:completed is deliberately NOT a unique state (same reasoning as AnnotationThumbnailJob): once a run finishes, a later request — a new dimension added to the kit, a replaced original, a run that failed to write an instance — must be able to enqueue again instead of being silently swallowed until the period lapses.