feat(drinks): add Sekt category with rainbow button styling

- Add Drink.Category.sekt for consignment alcoholic drinks
- Order sekt under "mit Alkohol" section, after Radler
- Rainbow gradient CSS for .drink-btn-sekt
- Rebalance all drink button brightness to match rainbow midpoint
- Seed Sekty Drink (3.50€, no deposit, no purchase price)
- Update Sternburg Export 10.99→9.49, Altenburger Helles 15.99→13.99

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gaehsnitz/migrations/0010_add_sekt_category.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.5 on 2026-06-09 13:42
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('gaehsnitz', '0009_anonymous_user'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='drink',
+            name='category',
+            field=models.CharField(choices=[('beer', 'Bier'), ('radler', 'Radler'), ('sekt', 'Sekt'), ('alc_free_beer', 'Bier alkoholfrei'), ('alc_free_radler', 'Radler alkoholfrei'), ('soft', 'Softdrink'), ('water', 'Wasser')], default='beer', max_length=16, verbose_name='Kategorie'),
+        ),
+    ]