feat: auto-populate slug fields on save

Add clubmanager.base.unique_slugify(instance, value, scope=...): slugify a
source value, truncate to the field's max_length, and append -2/-3/... to
stay unique within a scope. ClubScopedModel gains a slug_source hook that
fills a blank slug (unique per club) on save.

Wire it up so every SlugField auto-populates from its natural source when
left blank (explicit values are always kept):
- shop.Product.slug   <- name   (per club)
- formbuilder.Form.slug <- title (per club)
- formbuilder.Field.key <- label (per form)
Club.slug already auto-populated; refactor it onto the shared helper.

Also fix shop.Product.slug's multi-tenancy bug: it was globally unique
(unique=True); make it unique per club like the others. Migrations added.
Full suite at 100% coverage.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

shop/migrations/0001_initial.py

@@ -0,0 +1,29 @@
+# Generated by Django 6.0.6 on 2026-07-12 19:29
+
+import django.db.models.deletion
+import uuid
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = [
+        ('club', '0008_alter_clubmembership_unique_together_and_more'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Product',
+            fields=[
+                ('id', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
+                ('name', models.CharField(max_length=255, verbose_name='name')),
+                ('slug', models.SlugField(max_length=255, unique=True, verbose_name='slug')),
+                ('club', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='%(class)ss', to='club.club')),
+            ],
+            options={
+                'abstract': False,
+            },
+        ),
+    ]

shop/migrations/0002_alter_product_slug_and_more.py

@@ -0,0 +1,23 @@
+# Generated by Django 6.0.6 on 2026-07-12 19:39
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('club', '0008_alter_clubmembership_unique_together_and_more'),
+        ('shop', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='product',
+            name='slug',
+            field=models.SlugField(blank=True, max_length=255, verbose_name='slug'),
+        ),
+        migrations.AddConstraint(
+            model_name='product',
+            constraint=models.UniqueConstraint(fields=('club', 'slug'), name='unique_product_slug_per_club'),
+        ),
+    ]