Generate seasons per club instead of a hardcoded Aug-May window

Club now carries its own season_start and season_duration_months,
editable via controlpanel; generate_seasons chains each new season off
the day after the club's last one ends (or its configured start, for a
club with none yet) instead of assuming every club runs Aug 1 - May 31.

Adds --resync to the generate_seasons command to clean up seasons left
over from the old hardcoded rule -- removing any that don't match a
club's current settings and aren't still referenced by real data.

club/migrations/0017_club_season_duration_months_club_season_start.py

@@ -0,0 +1,25 @@
+# Generated by Django 6.0.6 on 2026-08-03 14:30
+
+import datetime
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('club', '0016_clubmembership_amount_paid_clubmembership_fee_amount_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='club',
+            name='season_duration_months',
+            field=models.PositiveSmallIntegerField(default=12, help_text='How many months a season lasts, counted from its start date.', validators=[django.core.validators.MinValueValidator(1), django.core.validators.MaxValueValidator(24)], verbose_name='season duration (months)'),
+        ),
+        migrations.AddField(
+            model_name='club',
+            name='season_start',
+            field=models.DateField(default=datetime.date(2000, 8, 1), help_text='Which day of the year a season begins — only the month and day are used, the year is ignored.', verbose_name='season start'),
+        ),
+    ]