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/migrations/0010_attendance_showed_up.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.6 on 2026-08-03 20:55
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('events', '0009_attendance_created_attendance_modified_event_created_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='attendance',
+            name='showed_up',
+            field=models.BooleanField(blank=True, default=None, help_text='Recorded by a check-in, separate from the RSVP status above. Blank means no check-in has been recorded yet.', null=True, verbose_name='showed up'),
+        ),
+    ]