1e8f79fe18
The web subtitle handler wrapped the downloaded-subtitle lookup in
`if err == nil { ... }`, so a real DB/backing-store failure from
ListDownloadedSubtitles was silently swallowed and control fell through
to a generic 404 ("Subtitle track not found") with no server-side signal.
This made genuine internal failures look like an intermittent client-side
"subtitles won't render" bug and left nothing in the logs to diagnose.
On error, log at ERROR (with file_id/track/error, matching the sibling
font-extraction path) and return 500 — mirroring the neighbouring error
handling in the same branch (S3 GetObject -> 502, ConvertToVTT -> 500).
The genuine not-found fall-through is preserved for the case where the
listing succeeds but the requested index is out of range.
Closes #248
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>