fix(auth): Google adoption revokes the password of a never-activated account

Without SMTP, registration marks the address verified with no proof, and an ADMIN_EMAILS /
ALLOWED_EMAILS address gets no invite row to gate it. So a stranger who registered such an address
(or a sibling spelling) before its owner kept their own password when the owner's first Google
sign-in adopted the row, activated it and made it admin, and could then sign in with it.

The adoption now treats a never-activated row like an unverified one: the password goes,
password_revoked_at is stamped and the session epoch bumped. An account already let in keeps its
password. The helper is renamed to say what it now judges (revoke_untrusted_preset_password).

test_google_callback.py drives the callback's account-matching branches through the real route,
none of which a test ran before: the squat (red on the old rule), adoption by mailbox, the
sub-collision and unverified-address refusals, the refused sign-in recorded in Google's locale, and
the link flow's conflict, mismatch and attach answers.

backend/app/models.py

@@ -109,7 +109,7 @@ class User(Base, TimestampMixin, CanonicalEmailMixin):
     # argon2id hash for email+password login; NULL for Google-only accounts.
     password_hash: Mapped[str | None] = mapped_column(String(255))
     # When SEC-10 dropped a preset password on Google adoption (see the single caller of
-    # auth.revoke_preset_password_if_unverified; admin approval no longer revokes). Without
+    # auth.revoke_untrusted_preset_password; admin approval no longer revokes). Without
     # this trace a revoked account was indistinguishable from a Google-only one, so it failed the
     # password-reset eligibility check and could never be recovered — it could neither sign in nor
     # reset. Set = "had a password, lost it to the guard" -> reset-eligible. Cleared whenever a