Add a reusable notification system; wire it into news publishing

New `notifications` app: Notification (club-scoped, keyed to the
member it's about, generic `source` via a ContentType/object_id pair
so future activities can reuse this without a new model each time)
plus notify_members(), which resolves each member's own email (if
they hold a login) and every parent/guardian's, always -- a child
with their own account doesn't opt their parents out -- and emails
the club-branded template to whichever addresses that resolves to.
Delivery is email-only for now (no in-app feed exists yet); the row
is created either way, ready for one later.

news.tasks.notify_news_published resolves the audience (a team-scoped
item's current rosters, or every active member if it's club-wide) and
calls notify_members with the item's title/plain-text body.
NewsPublishForm gained a "Notify linked members" checkbox (opt-in,
default off); when checked, NewsPublishView schedules the task with
Celery's `eta` set to the item's own published_at -- a scheduled
item's notification arrives when it actually goes live, and an
immediate publish (eta in the past) just runs right away, no separate
branch needed.

Registered the new notification email on the Club identity page's
Email tab alongside the others.

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

management/views.py

@@ -49,6 +49,7 @@ from members.models import Family, FamilyMembership, Group, GroupMembership, Mem
 from members.services.claims import ClaimError, approve_claim, children_awaiting_a_parent, reject_claim, send_claim_approved_email, suggested_children
 from members.services.family import add_child_to_family, add_parent_to_family, attach_to_family, detach_from_family, get_or_create_login_user, grant_login, register_family
 from news.models import News, NewsPhoto
+from news.tasks import notify_news_published
 from shop.models import Discount, Invoice, Order, Product
 from teams.models import Position, RefereeLevel, RefereeProfile, StaffAssignment, Team, TeamMembership, TeamPhoto
 from teams.services import eligible_roster_members
@@ -2195,6 +2196,12 @@ class NewsPublishView(NewsPublisherRequiredMixin, RedirectOnInvalidMixin, FormVi
         news_item = get_object_or_404(News.objects.filter(club=self.request.club), pk=self.kwargs["pk"])
         news_item.publish(at=form.cleaned_data["published_at"])
 
+        if form.cleaned_data["notify_members"]:
+            # eta in the past (the common, "publish now" case) just runs right
+            # away -- see news.tasks' own module docstring for why there's no
+            # separate immediate/scheduled branch here.
+            notify_news_published.apply_async(args=[str(news_item.pk)], eta=news_item.published_at)
+
         if news_item.is_scheduled:
             body = _("“%(news)s” is scheduled to go live on %(date)s.") % {"news": news_item, "date": news_item.published_at}
         else: