Under supports_model_type_tags, a model_type: edit on a filesystem-backed
model asset now relocates and re-registers the file to the folder matching
the new model_type:<folder_name>, so the file location stays coherent with
the label instead of a label-only relabel that left the file loader-orphaned.
Triggered off the POST /tags add (apply_tags): on a registered folder change
the file moves (preserving subfolder + filename), path-derived system tags
are re-derived, and filename metadata refreshed, all in one transaction with
file rollback on failure. Collisions are rejected (never clobber); an
unregistered model_type: stays a permissive label; non-model and hash-only
references are untouched. Shared on-disk folders keep their plural twins.
