New admin endpoint GET /api/models/storage reports disk usage of the
installed models: each model's files and sizes, files shared between
models counted once, and configured files that are missing from disk.
The models WebUI page shows a usage summary, each model's file table,
and the full file list with per-file status.
Assisted-by: Claude Code:claude-fable-5
Signed-off-by: Plamen K. Kosseff <p.kosseff@gmail.com>