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/server/api/console/v1/graphql/cookie_banner.graphql

@@ -119,6 +119,7 @@ type CookieCategory implements Node {
     id: ID!
     cookieBanner: CookieBanner @goField(forceResolver: true)
     name: String!
+    slug: String!
     description: String!
     kind: CookieCategoryKind!
     rank: Int!
@@ -300,6 +301,7 @@ input PublishCookieBannerVersionInput {
 input CreateCookieCategoryInput {
     cookieBannerId: ID!
     name: String!
+    slug: String!
     description: String!
     rank: Int!
 }
@@ -307,6 +309,7 @@ input CreateCookieCategoryInput {
 input UpdateCookieCategoryInput {
     cookieCategoryId: ID!
     name: String
+    slug: String
     description: String
 }