Add a team attendance KPI panel and no-show check-in tracking

Team pages now show, for the selected season: overall attendance rate,
best/worst attenders, players who missed the last 2 practices, and
no-shows (an affirmative RSVP checked in as absent). No-shows need a
real distinction the RSVP status alone can't make, so Attendance gains
a separate showed_up tri-state field, usable today via Django admin --
a full check-in screen is future work for the coaches app.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R1gj3J1QPfP38XWpnpbFpy

events/admin.py

@@ -58,7 +58,7 @@ class EventAdmin(admin.ModelAdmin):
 
 @admin.register(Attendance)
 class AttendanceAdmin(admin.ModelAdmin):
-    list_display = ["event", "member", "status"]
-    list_filter = ["status", "event__kind"]
+    list_display = ["event", "member", "status", "showed_up"]
+    list_filter = ["status", "showed_up", "event__kind"]
     search_fields = ["event__title", "member__first_name", "member__last_name"]
     raw_id_fields = ["event", "member"]