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>

.env.production.example

@@ -48,3 +48,14 @@ DJANGO_EMAIL_USE_TLS=True
 DJANGO_DEFAULT_FROM_EMAIL=RosterChief <noreply@rosterchief.app>
 # Where a club is told to reply with a billing question.
 ROSTERCHIEF_BILLING_CONTACT_EMAIL=billing@rosterchief.app
+
+# --- Email via Resend (resend.com), as an alternative to the SMTP block above ---
+# Two ways to use Resend; pick one, don't set both:
+#   1. Resend's SMTP relay -- no code involved, just point the SMTP settings above at it:
+#        DJANGO_EMAIL_HOST=smtp.resend.com
+#        DJANGO_EMAIL_HOST_USER=resend
+#        DJANGO_EMAIL_HOST_PASSWORD=<your Resend API key>
+#   2. Resend's HTTP API via rosterchief.mail.ResendEmailBackend (see that module):
+#        DJANGO_EMAIL_BACKEND=rosterchief.mail.ResendEmailBackend
+#        RESEND_API_KEY=<your Resend API key>
+RESEND_API_KEY=