Add /models/prune

[doringeman]

On top of #46.

E.g.,

$ MODEL_RUNNER_HOST=http://localhost:8080 docker model pull ghcr.io/ilopezluna/smollm2
$ MODEL_RUNNER_HOST=http://localhost:8080 docker model ls
MODEL NAME                  PARAMETERS  QUANTIZATION  ARCHITECTURE  MODEL ID      CREATED      SIZE
ghcr.io/ilopezluna/smollm2  134.52 M    Q2_K          llama         425de603d029  6 weeks ago  82.41 MiB
$ MODEL_RUNNER_HOST=http://localhost:8080 docker model prune
WARNING! This will remove the entire models directory.
Are you sure you want to continue? [y/N] y
$ MODEL_RUNNER_HOST=http://localhost:8080 docker model ls
MODEL NAME  PARAMETERS  QUANTIZATION  ARCHITECTURE  MODEL ID  CREATED  SIZE