feat: Todo altijd gekoppeld aan product backlog

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

actions/todos.ts

@@ -14,11 +14,20 @@ async function getSession() {
 export async function createTodoAction(_prevState: unknown, formData: FormData) {
   const session = await getSession()
   if (!session.userId) return { error: 'Niet ingelogd' }
+  if (session.isDemo) return { error: 'Niet beschikbaar in demo-modus' }
 
   const title = (formData.get('title') as string)?.trim()
-  if (!title) return { error: 'Titel is verplicht' }
+  const productId = (formData.get('productId') as string)?.trim()
 
-  await prisma.todo.create({ data: { user_id: session.userId, title } })
+  if (!title) return { error: 'Titel is verplicht' }
+  if (!productId) return { error: 'Product is verplicht' }
+
+  const product = await prisma.product.findFirst({
+    where: { id: productId, user_id: session.userId, archived: false },
+  })
+  if (!product) return { error: 'Product niet gevonden' }
+
+  await prisma.todo.create({ data: { user_id: session.userId, product_id: productId, title } })
   revalidatePath('/todos')
   return { success: true }
 }