Correcting the earlier scope-union-only fix. The real damage from a logout→login isn't
just the stored `scope` field — verified via a live token refresh that the stored refresh
token itself had been REPLACED with a base-only one: a plain sign-in requests only
BASE_SCOPES and Google handed back a fresh base-only access+refresh token, which
_store_token adopted verbatim, destroying the read/write grant (the old refresh token
stays valid on Google's side, so overwriting it is what loses access).
_store_token now detects when an exchange would NARROW our YouTube scopes and, in that
case, keeps the WHOLE existing grant (refresh token, access token, scopes) untouched.
Broader-or-equal exchanges (first grant, read→write upgrade) adopt + union as before.
Unit-verified across base-relogin / upgrade / new-user / base-user cases.
9e6c90bcaf