feat(prisma, src): add slug field to Prompt model and implement slug generation for prompts without slugs

prisma/migrations/20251216124600_add_prompt_slug/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "prompts" ADD COLUMN "slug" TEXT;

prisma/schema.prisma

@@ -76,6 +76,7 @@ model VerificationToken {
 model Prompt {
   id                  String             @id @default(cuid())
   title               String
+  slug                String?
   description         String?
   content             String
   type                PromptType         @default(TEXT)