DESIGN.md §15 as amended: a language is satisfied by exactly one
sidecar, and no filename segment distinguishes forced from plain from
SDH. A unique index over sidecar rows says so; embedded rows keep their
own key, since several tracks for one language can legitimately coexist
inside a video.
Existing databases may hold a duplicate from a manual grab that beat the
API's path check, so the migration resolves them rather than failing: a
real subtitle beats a machine translation, and of two of the same kind
the newest wins. The files stay on disk for the manual delete to clean
up.
`record_file` no longer swallows every conflict — only the two that mean
"arr already knows this file".
Miguel Palhas