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>

clubmanager/base.py

@@ -2,6 +2,7 @@ import uuid
 from typing import TYPE_CHECKING
 
 from django.db import models
+from django.utils.text import slugify
 
 from club.tenancy import require_current_club
 
@@ -9,6 +10,30 @@ if TYPE_CHECKING:
     from club.models import Club
 
 
+def unique_slugify(instance, value, *, slug_field="slug", scope=None):
+    """Return a slug derived from ``value``, unique within ``scope``.
+
+    Truncates to the slug field's ``max_length`` and appends ``-2``, ``-3``, …
+    on collision. ``scope`` is a dict of field lookups the uniqueness is
+    checked within (e.g. ``{"club": club}`` for per-club, ``{}``/``None`` for
+    global).
+    """
+    max_length = instance._meta.get_field(slug_field).max_length
+    base = slugify(value)[:max_length] or "item"
+
+    queryset = type(instance)._default_manager.exclude(pk=instance.pk)
+    if scope:
+        queryset = queryset.filter(**scope)
+
+    slug = base
+    suffix = 2
+    while queryset.filter(**{slug_field: slug}).exists():
+        tail = f"-{suffix}"
+        slug = f"{base[: max_length - len(tail)]}{tail}"
+        suffix += 1
+    return slug
+
+
 class TenantQuerySet(models.QuerySet):
     def for_club(self, club: Club):
         return self.filter(club=club)
@@ -32,6 +57,10 @@ class ClubScopedModel(UUIDModel):
     club = models.ForeignKey("club.Club", on_delete=models.CASCADE, related_name="%(class)ss")
     objects = TenantQuerySet.as_manager()
 
+    # Subclasses with a ``slug`` field set this to the source field name (e.g.
+    # "name"/"title") to auto-populate the slug — unique per club — on save.
+    slug_source = None
+
     class Meta:
         abstract = True
 
@@ -39,4 +68,7 @@ class ClubScopedModel(UUIDModel):
         if self.club_id is None:
             self.club = require_current_club()
 
+        if self.slug_source and not self.slug:
+            self.slug = unique_slugify(self, getattr(self, self.slug_source), scope={"club": self.club})
+
         super().save(*args, **kwargs)