Add slug to cookie categories for stable consent identifiers

The category slug provides a stable, URL-safe key used as the
data-cookie-consent attribute value and consent data key, replacing
the fragile category name. This prevents breakage when categories
are renamed.

- Add slug column with unique-per-banner constraint and backfill migration
- Add Slug validator (lowercase alphanumeric + hyphens)
- Propagate slug through GraphQL schema, service layer, and snapshot
- Update console UI with slug field in create/edit forms
- Switch cookie-banner widget to use slug as consent data keys

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/cookiebanner/errors.go

@@ -26,6 +26,7 @@ var (
 	ErrNoPublishedVersion         = errors.New("no published cookie banner version")
 	ErrNoDraftVersion             = errors.New("no draft cookie banner version to publish")
 	ErrCannotDeleteSystemCategory = errors.New("cannot delete system cookie category")
+	ErrCategorySlugAlreadyExists  = errors.New("a category with this slug already exists in this banner")
 	ErrOriginAlreadyInUse         = errors.New("origin is already used by another active cookie banner")
 	ErrConsentNotFound            = errors.New("consent record not found")
 	ErrCookieNotFound             = errors.New("cookie not found")