Let eligible referees self-serve sign-up for games needing one

Creating (or re-teaming) a home game for a club-managed team now auto-
invites every eligible referee (teams.RefereeProfile) via a new
RefereeSignup model, notifying them the same way news/events already do.
They see it as its own distinct row on the mobile Calendar (own accent
colour) and can accept or decline right there -- accepting routes through
the existing capacity-checked assign_referee (assigned_by=None marks it
self-service), so it lands as a real EventReferee row with no separate sync
step. The desktop referee-management screen and event detail page both
surface pending invites and flag self-signed-up referees distinctly from
admin assignments.

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

mobile/urls.py

@@ -14,6 +14,7 @@ urlpatterns = [
     # Member mode (M1-M7).
     path("", views.HomeView.as_view(), name="home"),
     path("calendar/", views.CalendarView.as_view(), name="calendar"),
+    path("referee-signups/<uuid:signup_id>/respond/", views.RefereeSignupRespondView.as_view(), name="referee_signup_respond"),
     path("events/<uuid:pk>/", views.EventDetailView.as_view(), name="event_detail"),
     path("news/", views.NewsListView.as_view(), name="news_list"),
     path("news/<slug:slug>/", views.NewsDetailView.as_view(), name="news_detail"),