Fix time-dependent briefing dispatch test that fails in CI overnight

The test_generate_all_dispatches_for_eligible test relied on wall clock
time, only passing when CI ran after 8:25 AM Eastern. Mock datetime in
the tasks module to pin UTC time to 20:00 (3 PM ET), ensuring the
morning slot generation_time check always passes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: samuelclay

apps/briefing/tests.py

@@ -1779,12 +1779,27 @@ def test_generate_all_skips_if_locked(self, mock_redis_cls):
         GenerateBriefings()
         # tests.py: If lock is held, nothing else should happen
 
+    @patch("apps.briefing.tasks.datetime")
     @patch("apps.briefing.models.MBriefing")
     @patch("apps.briefing.tasks.GenerateUserBriefing")
     @patch("redis.Redis")
-    def test_generate_all_dispatches_for_eligible(self, mock_redis_cls, mock_user_task, mock_mbriefing):
+    def test_generate_all_dispatches_for_eligible(self, mock_redis_cls, mock_user_task, mock_mbriefing, mock_dt):
+        import datetime as real_dt
+
+        import pytz
+
         from apps.briefing.tasks import GenerateBriefings
 
+        # tests.py: Pin to 3 PM ET (20:00 UTC) so the morning slot check always passes
+        fake_utc = real_dt.datetime(2026, 3, 5, 20, 0, 0)
+        mock_dt.datetime.utcnow.return_value = fake_utc
+        mock_dt.datetime.now.side_effect = lambda tz=None: (
+            pytz.utc.localize(fake_utc).astimezone(tz) if tz else fake_utc
+        )
+        mock_dt.datetime.combine = real_dt.datetime.combine
+        mock_dt.timedelta = real_dt.timedelta
+        mock_dt.time = real_dt.time
+
         mock_r = MagicMock()
         mock_redis_cls.return_value = mock_r
         mock_r.set.return_value = True