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>

formbuilder/migrations/0004_alter_field_key_alter_form_slug.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 = [
+        ('formbuilder', '0003_fix_tenant_uniqueness'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='field',
+            name='key',
+            field=models.SlugField(blank=True, verbose_name='key'),
+        ),
+        migrations.AlterField(
+            model_name='form',
+            name='slug',
+            field=models.SlugField(blank=True, verbose_name='slug'),
+        ),
+    ]