fix(auth): a cancel at Google lands where the browser is; the probe says why it refused

A refused session keeps its cookie, so the cancel branch, keyed on the cookie's account, sent a
signed-out browser to the in-app toast the login page never shows: it now asks whether the
session holds. The probe's log line named every refusal 'its account no longer holds'; it now
carries the reason. The pruning docstring says again that a renewal may land a request's time
after the revoke, and the test pins the day of margin. The test clock clamps in the zone asked for.

backend/tests/test_test_clock.py

@@ -49,3 +49,26 @@ def test_a_test_reads_the_clock_the_app_stamps_with(monkeypatch):
     monkeypatch.setattr(conftest._NeverBack, "last", None)
     stamped = diagnostics.datetime.now(timezone.utc)
     assert datetime.now(timezone.utc) >= stamped
+
+
+def test_a_clamped_reading_is_in_the_zone_asked_for(monkeypatch):
+    # The Pacific quota day read through a clamp came back in UTC: its date was the next day's.
+    from zoneinfo import ZoneInfo
+
+    import conftest
+
+    pacific = ZoneInfo("America/Los_Angeles")
+    utc_reading = datetime(2099, 1, 2, 3, 0, 0, tzinfo=timezone.utc)  # 1 January 19:00 in Pacific
+    stepped_back = datetime(2099, 1, 1, 18, 59, 59, tzinfo=pacific)  # a second earlier
+
+    class Stepping(datetime):
+        @classmethod
+        def now(cls, tz=None):
+            return utc_reading if tz is timezone.utc else stepped_back
+
+    monkeypatch.setattr(conftest._NeverBack, "clock", Stepping)
+    monkeypatch.setattr(conftest._NeverBack, "last", None)
+    conftest._NeverBack.now(timezone.utc)
+    clamped = conftest._NeverBack.now(pacific)
+    assert clamped.tzinfo is pacific
+    assert clamped.date().isoformat() == "2099-01-01"