DeleteUserWithCascadeAsync never removed UserPermissionOverrides rows, and
the FK on UserId is Restrict. Once an override was saved for a member, the
admin hard-delete (DeleteUserAsync -> DeleteUserWithCascadeAsync) failed the
foreign key inside the transaction and the controller surfaced it as a 400,
so the user was never deleted. Add an explicit ExecuteDelete on
UserPermissionOverrides before the user is removed, matching how UserRoles is
already cleared in the same method (no schema change).
SetOverrideAsync was check-then-insert on the composite key with no
DbUpdateException handling, so two concurrent PUTs for the same
(UserId, PermissionKey) let the loser violate PK_UserPermissionOverrides and
return 500. Catch the conflict, detach the pending insert, and converge by
updating the persisted row to the caller's requested state.
