feat(remoteapi): auto-create databases on first push to own namespace

Push-to-create: an authenticated, non-suspended caller pushing (or
cloning) an unknown repo under their OWN namespace has it transparently
created — a PRIVATE repository row plus a genuinely empty on-disk NBS
store — then proceeds through the normal ACL check as the owner. Any
other case (anonymous, suspended, another user's namespace, invalid
name) still returns NotFound, so a stranger's namespace is never leaked
and nothing is created.

storage.InitEmptyStore creates the store WITHOUT WriteEmptyRepo: an
"Initialize data repository" commit would make the client's first push a
non-fast-forward and be rejected. An empty store (root = empty hash) lets
the initial push land as the repo's first history. The interceptor
auto-create is race-safe (ErrNameTaken re-fetch) and rolls the row back
if the store cannot be created.

Proven end-to-end (integration): a real `dolt push` to a new name
auto-creates PRIVATE + fast-forwards + re-clones; a foreign-namespace
push is denied with no row created. All prior clone/push/ACL scenarios
still pass.