Add a secondary brand colour for clubs

Mirrors primary_color: a club-picked hex highlight used for accents like
the avatar-initials badge, with a computed readable text colour so a
pale pick doesn't produce white-on-yellow text. The shared contrast math
moves into _content_color_for so both colours use the same rule.

club/migrations/0014_club_secondary_color.py

@@ -0,0 +1,19 @@
+# Generated by Django 6.0.6 on 2026-07-24 16:30
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('club', '0013_club_created_club_modified_clubmembership_created_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='club',
+            name='secondary_color',
+            field=models.CharField(blank=True, help_text="Hex colour for highlights on the club's pages, e.g. avatar initials. Defaults to the theme's secondary colour.", max_length=7, validators=[django.core.validators.RegexValidator('^#[0-9a-fA-F]{6}$', 'Enter a colour as a hex value, e.g. #be185d.')], verbose_name='secondary colour'),
+        ),
+    ]