[Feature] Surface a game's soundtrack on its sibling entries

Is your feature request related to a problem? Please describe. A game owned on two platforms is two ROM entries, and RomM already links them as siblings. A soundtrack lives in one entry's soundtrack/ folder, so only that entry has music and the sibling shows none. Both workarounds are poor: pick one platform and let the other entry look empty, or copy the files under both — which doubles the disk usage and, because the Jukebox plays every soundtrack in the library as one collection, lists every track twice.

Describe the solution you'd like Let a sibling's soundtrack be reachable from an entry that has none — on the details page and through the music API — marked as coming from the sibling rather than silently merged into it.

Describe alternatives you've considered Hard-linking the files, so the disk cost disappears. The duplicate entries remain: each file is still its own RomFile row, so the Jukebox still lists every track twice.

Additional context Merging automatically would be wrong as a blanket rule — a game's music can genuinely differ between platform versions, so two sibling soundtracks are not always the same recording. Surfacing the sibling's tracks, or making the link explicit, keeps that case intact where merging by assumption would not.