Add group/club-wide event audiences and a Resend email backend

Events can now target members.Group audiences alongside teams, or go
club_wide (every ACTIVE ClubMembership member for the event's season)
instead of specific teams/groups -- the two are mutually exclusive,
enforced in EventForm/EventSeriesForm.clean() since an M2M can't be
validated via a DB CheckConstraint or Event.clean() (no PK yet). Attendance
sync (events/signals.py) now reacts to GroupMembership and ClubMembership
changes the same way it already did for TeamMembership. Authorization:
club.services.access.groups_manageable_by mirrors teams_managed_by (all
groups for an ADMIN, else only the ones the user belongs to -- Group has no
manager/owner concept); a non-admin needs at least one managed team or
belonged-to group to create/edit an event, club_wide stays admin-only, and
EventManagerRequiredMixin gained a get_groups() hook so a non-admin who
creates a group-only event isn't immediately locked out of managing it.

Also adds rosterchief.mail.ResendEmailBackend, an HTTP-API-based Django
email backend for Resend (resend.com) using the existing `requests`
dependency -- no new SDK. Opt in via DJANGO_EMAIL_BACKEND and RESEND_API_KEY;
every Django-sent email (allauth's password reset included) follows
whichever EMAIL_BACKEND is configured, so this covers all of them for free.
Resend's own SMTP relay remains a valid code-free alternative, documented
alongside it in .env.production.example.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

events/signals.py

@@ -1,11 +1,15 @@
 """Signal wiring that keeps attendance in sync with the event audience.
 
-Registered from ``EventsConfig.ready``. Two triggers:
+Registered from ``EventsConfig.ready``. Triggers:
 
-* editing an event (its ``start``, or its ``teams`` / ``invited_members`` /
-  ``excluded_members``) re-syncs that event;
+* editing an event (its ``start``, or its ``teams`` / ``groups`` /
+  ``invited_members`` / ``excluded_members``) re-syncs that event;
 * adding or removing a member from a team roster re-syncs that team's future
-  events.
+  events;
+* adding or removing a member from a group re-syncs that group's future
+  events;
+* a club membership going active/inactive re-syncs every future club_wide
+  event for that club.
 """
 
 from django.core.exceptions import ValidationError
@@ -14,8 +18,10 @@ from django.dispatch import receiver
 from django.utils import timezone
 from django.utils.translation import gettext_lazy as _
 
+from club.models import ClubMembership
 from events.models import Event, EventSeries
 from events.services import sync_event_attendances
+from members.models import Group, GroupMembership
 from teams.models import Team, TeamMembership
 
 M2M_SYNC_ACTIONS = {"post_add", "post_remove", "post_clear"}
@@ -32,12 +38,22 @@ def validate_teams_same_club(sender, instance, action, reverse, pk_set, **kwargs
         raise ValidationError(_("Teams must belong to the same club as the event."))
 
 
+@receiver(m2m_changed, sender=Event.groups.through)
+@receiver(m2m_changed, sender=EventSeries.groups.through)
+def validate_groups_same_club(sender, instance, action, reverse, pk_set, **kwargs):
+    if action != "pre_add" or reverse:
+        return
+    if Group.objects.filter(pk__in=pk_set).exclude(club_id=instance.club_id).exists():
+        raise ValidationError(_("Groups must belong to the same club as the event."))
+
+
 @receiver(post_save, sender=Event)
 def sync_on_event_save(sender, instance, **kwargs):
     sync_event_attendances(instance)
 
 
 @receiver(m2m_changed, sender=Event.teams.through)
+@receiver(m2m_changed, sender=Event.groups.through)
 @receiver(m2m_changed, sender=Event.invited_members.through)
 @receiver(m2m_changed, sender=Event.excluded_members.through)
 def sync_on_audience_change(sender, instance, action, reverse, model, pk_set, **kwargs):
@@ -58,3 +74,19 @@ def sync_on_roster_change(sender, instance, **kwargs):
     now = timezone.now()
     for event in Event.objects.filter(teams=instance.team_id, start__gte=now).distinct():
         sync_event_attendances(event)
+
+
+@receiver(post_save, sender=GroupMembership)
+@receiver(post_delete, sender=GroupMembership)
+def sync_on_group_membership_change(sender, instance, **kwargs):
+    now = timezone.now()
+    for event in Event.objects.filter(groups=instance.group_id, start__gte=now).distinct():
+        sync_event_attendances(event)
+
+
+@receiver(post_save, sender=ClubMembership)
+@receiver(post_delete, sender=ClubMembership)
+def sync_on_club_membership_change(sender, instance, **kwargs):
+    now = timezone.now()
+    for event in Event.objects.filter(club_wide=True, club_id=instance.club_id, start__gte=now).distinct():
+        sync_event_attendances(event)