feat(ST-l9kkxh2m): CLI-script scripts/create-admin.ts voor admin-bootstrap

- Maakt user aan als die niet bestaat, anders upgrade bestaande user
- Upsert ADMIN in user_roles (idempotent)
- Helder foutbericht als argumenten ontbreken (process.exit(1))
- package.json scripts: "create-admin": "tsx scripts/create-admin.ts"

package.json

@@ -15,6 +15,7 @@
     "db:erd": "prisma generate",
     "db:erd:watch": "chokidar \"prisma/schema.prisma\" -c \"npm run db:erd\"",
     "db:insert-milestone": "tsx scripts/insert-milestone.ts",
+    "create-admin": "tsx scripts/create-admin.ts",
     "seed": "prisma db seed",
     "docs:index": "node scripts/generate-docs-index.mjs",
     "docs:check-links": "node scripts/check-doc-links.mjs",