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/migrations/0022_event_club_wide_event_groups_eventseries_club_wide_and_more.py

@@ -0,0 +1,34 @@
+# Generated by Django 6.0.6 on 2026-08-11 09:28
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('events', '0021_eventreferee_external_name_eventreferee_fee_and_more'),
+        ('members', '0004_group_groupmembership_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='event',
+            name='club_wide',
+            field=models.BooleanField(default=False, help_text="Invite every active club member for this event's season, instead of specific teams/groups. Can't be combined with teams or groups.", verbose_name='whole club'),
+        ),
+        migrations.AddField(
+            model_name='event',
+            name='groups',
+            field=models.ManyToManyField(blank=True, help_text='Invite every current member of these groups.', related_name='scheduled_events', to='members.group', verbose_name='groups'),
+        ),
+        migrations.AddField(
+            model_name='eventseries',
+            name='club_wide',
+            field=models.BooleanField(default=False, help_text="Invite every active club member for each occurrence's season, instead of specific teams/groups. Can't be combined with teams or groups.", verbose_name='whole club'),
+        ),
+        migrations.AddField(
+            model_name='eventseries',
+            name='groups',
+            field=models.ManyToManyField(blank=True, help_text='Invite every current member of these groups.', related_name='event_series', to='members.group', verbose_name='groups'),
+        ),
+    ]