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>

club/models.py

@@ -2,10 +2,9 @@ import datetime
 
 from django.db import models
 from django.utils import timezone
-from django.utils.text import slugify
 from django.utils.translation import gettext_lazy as _
 
-from clubmanager.base import ClubScopedModel, UUIDModel
+from clubmanager.base import ClubScopedModel, UUIDModel, unique_slugify
 from members.models import Member
 
 
@@ -33,19 +32,9 @@ class Club(UUIDModel):
 
     def save(self, *args, **kwargs):
         if not self.slug:
-            self.slug = self._unique_slug()
+            self.slug = unique_slugify(self, self.name)
         super().save(*args, **kwargs)
 
-    def _unique_slug(self):
-        base = slugify(self.name) or "club"
-        slug = base
-        suffix = 2
-        existing = Club.objects.exclude(pk=self.pk)
-        while existing.filter(slug=slug).exists():
-            slug = f"{base}-{suffix}"
-            suffix += 1
-        return slug
-
 
 class Season(ClubScopedModel):
     start_date = models.DateField(_("start date"))