Scalability: Split `videos/<mediaid>` into `videos/<prefix>/<mediaid>` for more scalable storage on disk

Idea: To make this backwards compatible, store mediaid as `.` in database for files that are store in the new two-level hierarchy.

(`prefix` would be two first characters of the rest of the `mediaid`)

Start by locating the code that persists mediaid values and maps them to videos/ paths, then trace how existing media is read from disk and the database. Done means new files use the two-level prefix/mediaid hierarchy while existing records and files remain readable through the proposed backwards-compatible representation.

Written by the indexing model from the issue text.