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/coredata/cookie_banner_version.go

@@ -50,6 +50,7 @@ type (
 
 	CookieBannerVersionSnapshotCategory struct {
 		Name        string             `json:"name"`
+		Slug        string             `json:"slug"`
 		Description string             `json:"description"`
 		Kind        CookieCategoryKind `json:"kind"`
 		Cookies     CookieItems        `json:"cookies"`