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>

club/services/access.py

@@ -22,7 +22,7 @@ from django.utils import timezone
 from authentication.models import User
 from club.models import Club, ClubRole, Season
 from events.models import Event
-from members.models import FamilyMembership, Member
+from members.models import FamilyMembership, Group, Member
 from teams.models import StaffAssignment, Team
 
 #: Derived (never stored) roles.
@@ -92,6 +92,15 @@ def teams_managed_by(user: User, club: Club) -> QuerySet[Team]:
     ).distinct()
 
 
+def groups_manageable_by(user: User, club: Club) -> QuerySet[Group]:
+    """Groups the user may schedule an event for: all for an ADMIN, else only
+    the ones they're themselves a member of -- unlike Team, Group has no
+    manager/owner concept, so membership is the only claim there is to check."""
+    if is_club_admin(user, club):
+        return Group.objects.filter(club=club)
+    return Group.objects.filter(club=club, memberships__member__user=user).distinct()
+
+
 def teams_staffed_by(user: User, club: Club) -> QuerySet[Team]:
     """Teams the user is on the staff of this season, management or not.