Vellum had no subtitle support of any kind. This is the storage half.
Three sources, and they are genuinely different shapes: a sidecar file beside
the video has a path, an embedded stream has an index into the container and
no file of its own, and a downloaded track has a path but is regenerable.
stream_index is -1 for the file-backed sources rather than NULL, and that is
deliberate: NULL is never equal to NULL in SQL, so a unique index over a
nullable column would happily accept the same sidecar twice. With -1 the
UNIQUE (item_id, source, path, stream_index) actually does its job and a
rescan can upsert.
Nothing here is authoritative about the media. Deleting every row and
rescanning reproduces it exactly, which is what lets a vanished subtitle file
have its row removed - unlike a vanished video, which is marked instead
(ADR-003) because progress hangs off it and cannot be regenerated.
Verified on both paths: a real database at version 13 upgraded to 14 by an
older binary-then-newer sequence, and a fresh database applied 0001..0014.
