Failed downloads stored the raw yt-dlp/ffmpeg text and dumped it as a truncated red
one-liner in the queue. Classify the failure into a category (unavailable / login_required /
geo_blocked / format_unavailable / drm / postprocess / source_missing / network / unknown)
and surface it properly:
- backend: app/downloads/errors.classify maps the raw message to a category code; the worker
stores it in the new download_jobs.error_code (migration 0061) at both failure sites; resume
clears it; the serializer exposes it. Raw text kept as technical detail.
- frontend: the queue row shows a clean localized reason + a "details" link opening a modal
with the explanation, the raw text behind a disclosure, and a Retry only for transient
categories (network/postprocess/unknown).
- tests: classify() unit coverage over verbatim yt-dlp/ffmpeg message shapes.