Test the tagged-node-survives-user-deletion scenario at two layers:
DB layer (users_test.go):
- success_user_only_has_tagged_nodes: tagged nodes with nil
user_id do not block user deletion and survive it
- error_user_has_tagged_and_owned_nodes: user-owned nodes
still block deletion even when tagged nodes coexist
App layer (grpcv1_test.go):
- TestDeleteUser_TaggedNodeSurvives: full registration flow
with tagged PreAuthKey verifies nil UserID after registration,
absence from nodesByUser index, user deletion succeeds, and
tagged node remains in global node list
Also update auth_tags_test.go assertions to expect nil UserID
on tagged nodes, consistent with the new invariant.
Updates #3077
1e4fc3f179