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

events/admin.py

@@ -2,7 +2,7 @@ from django import forms
 from django.contrib import admin
 from django.utils.translation import gettext_lazy as _
 
-from .models import Attendance, Competition, Event, EventReferee, EventSeries, Lineup, LineupSelection, Location, Opponent
+from .models import Attendance, Competition, Event, EventReferee, EventSeries, Lineup, LineupSelection, Location, Opponent, RefereeSignup
 
 
 @admin.register(Opponent)
@@ -138,3 +138,11 @@ class EventRefereeAdmin(admin.ModelAdmin):
     @admin.display(description=_("total payable"))
     def total_payable(self, obj):
         return obj.total_payable
+
+
+@admin.register(RefereeSignup)
+class RefereeSignupAdmin(admin.ModelAdmin):
+    list_display = ["event", "member", "status", "responded_at"]
+    list_filter = ["status"]
+    search_fields = ["event__title", "member__first_name", "member__last_name"]
+    raw_id_fields = ["event", "member"]