A MediaAsset is unique on (source_kind, source_ref, format_sig), so the same video in
two formats is two assets — but rel_path keyed only on video_id, so they computed the
same path: the second download silently overwrote the first, and deleting either
removed the shared file. rel_path now takes an optional asset_id and appends "_a{id}"
(which Plex ignores — the .nfo sidecar carries the metadata); the worker passes the
asset id. Only affects NEW downloads (existing assets keep their stored rel_path).
Covered by a new test (two ids → two paths; omitted → the plain name, unchanged).