Notify members when a new event is planned

New events.tasks.notify_new_event, scheduled from
management.views.EventCreateView.form_valid -- the deliberate "a staff
member planned one new event" action, not every Event row that happens
to get created. A recurring series' rolling-horizon extension
(extend_event_series) and bulk fixture imports are NOT wired to this on
purpose: either would flood everyone with one push per occurrence
instead of the single, deliberate action this is meant to catch.

Notifies whoever is still NO_RESPONSE right after creation -- exactly
"everyone who needs to respond", using the existing notify_members() ->
Notification -> mobile.signals push chain already in place, so this is
in-app (visible in the M7 inbox, mark read/mark-all-read) and a push
notification both, with no new plumbing needed there.

Generalized mobile.views.NotificationsView's News-only "tap to open the
source" handling (previously isinstance(source, News)) into
_notification_source_link, covering Event sources too -- tapping an
"new event" notification now marks it read and opens the event's answer
screen, the same way a news notification already opened the article.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ECGMEwrc2k4D8VQuwjstj9

management/tests.py

@@ -5158,6 +5158,18 @@ class EventManagementTests(ManagementTestBase):
         self.assertRedirects(response, reverse("management:event_detail", args=[event.pk]))
         self.assertIn(self.own_team, event.teams.all())
 
+    def test_creating_an_event_notifies_the_invited_roster(self):
+        position = Position.objects.create(club=self.club, name="Forward", short_name="F")
+        player = Member.objects.create(first_name="Poul", last_name="Player")
+        TeamMembership.objects.create(team=self.own_team, member=player, season=self.season, position=position)
+        self.client.force_login(self.own_team_coach)
+
+        self.club_post("event_create", self.event_data())
+
+        event = Event.objects.get(title="Training")
+        notification = Notification.objects.get(member=player, content_type__model="event", object_id=str(event.pk))
+        self.assertEqual(notification.title, "Training")
+
     def test_creating_an_event_with_a_same_club_location_does_not_raise_a_cross_club_error(self):
         # Regression: Event.clean() rejects a location from another club by
         # comparing against self.club_id, which was still None on a brand-new