feat(events): formal end date for recurring series

Add EventSeries.until: an explicit series end that caps occurrence
generation (whichever comes first — it, the generation horizon, or the
rule's own COUNT/UNTIL). Blank means open-ended. Surface it in the admin.
100% coverage.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

events/tests.py

@@ -218,6 +218,15 @@ class OccurrenceExpansionTests(RecurrenceTestBase):
         self.assertNotIn(skipped, dts)
         self.assertEqual(len(dts), 3)
 
+    def test_series_until_caps_expansion(self):
+        series = self.make_series(rrule="FREQ=WEEKLY", until=self.anchor + timedelta(days=10))
+
+        dts = occurrence_datetimes(series, self.anchor + timedelta(days=90))
+
+        # Only the anchor and the first weekly repeat fall on/before `until`.
+        self.assertEqual(dts, [self.anchor, self.anchor + timedelta(weeks=1)])
+        self.assertTrue(all(dt <= series.until for dt in dts))
+
 
 class GenerateOccurrencesTests(RecurrenceTestBase):
     def test_materialises_occurrences_with_template_and_attendance(self):
@@ -245,6 +254,13 @@ class GenerateOccurrencesTests(RecurrenceTestBase):
 
         self.assertEqual(series.occurrences.count(), 4)
 
+    def test_generation_stops_at_series_until(self):
+        series = self.make_series(rrule="FREQ=WEEKLY", until=self.anchor + timedelta(days=10))
+
+        generate_occurrences(series)
+
+        self.assertEqual(series.occurrences.count(), 2)
+
     def test_gathering_and_deadline_come_from_offsets(self):
         series = self.make_series(gathering_offset=timedelta(minutes=30), deadline_offset=timedelta(days=1))