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/models.py

@@ -2,15 +2,17 @@ from django.db import models
 from django.db.models import UniqueConstraint
 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
 
 
 class Form(ClubScopedModel):
     title = models.CharField(_("title"), max_length=255)
-    slug = models.SlugField(_("slug"))
+    slug = models.SlugField(_("slug"), blank=True)
     description = models.TextField(_("description"), blank=True)
 
+    slug_source = "title"
+
     is_active = models.BooleanField(_("is active?"), default=True)
     login_required = models.BooleanField(_("login required?"), default=False)
 
@@ -43,7 +45,7 @@ class Field(UUIDModel):
         FILE = "file", _("file")
 
     form = models.ForeignKey(Form, on_delete=models.CASCADE, related_name="fields", verbose_name=_("form"))
-    key = models.SlugField(_("key"))
+    key = models.SlugField(_("key"), blank=True)
     label = models.CharField(_("label"), max_length=255)
     field_type = models.CharField(_("field type"), max_length=255, choices=FieldType.choices, default=FieldType.TEXT)
     required = models.BooleanField(_("required?"), default=True)
@@ -60,6 +62,11 @@ class Field(UUIDModel):
             UniqueConstraint(fields=["form", "key"], name="unique_field_key_per_form"),
         ]
 
+    def save(self, *args, **kwargs):
+        if not self.key:
+            self.key = unique_slugify(self, self.label, slug_field="key", scope={"form": self.form})
+        super().save(*args, **kwargs)
+
     def __str__(self):
         return self.label