refactor(dnd): remove drag-and-drop reorder for stories and tasks

- Remove reorderStoriesAction, reorderTasksAction, reorderSprintStoriesAction
- Delete REST route app/api/stories/[id]/tasks/reorder/route.ts
- Remove DnD from backlog story-panel and task-panel (flat list)
- Remove reorder-within-sprint branch from sprint-board-client handleDragEnd
- Switch SortableSprintRow to plain SprintRow using useDraggable (membership drag kept)
- Remove all DnD from task-list (status toggle + edit kept)
- Remove story-order/task-order/sprint-story-order/sprint-task-order mutation types and store handlers
- Remove related tests for deleted reorder route; fix sprint store tests

actions/tasks.ts

@@ -322,22 +322,3 @@ export async function updateTaskPlanAction(taskId: string, productId: string, im
   return { success: true }
 }
 
-export async function reorderTasksAction(storyId: string, orderedIds: string[]) {
-  const session = await getSession()
-  if (!session.userId) return { error: 'Niet ingelogd' }
-  if (session.isDemo) return { error: 'Niet beschikbaar in demo-modus' }
-
-  const story = await prisma.story.findFirst({
-    where: { id: storyId, product: productAccessFilter(session.userId) },
-  })
-  if (!story) return { error: 'Story niet gevonden' }
-
-  await prisma.$transaction(
-    orderedIds.map((id, i) =>
-      prisma.task.update({ where: { id }, data: { sort_order: i + 1.0 } })
-    )
-  )
-
-  revalidatePath(`/products/${story.product_id}/sprint/planning`)
-  return { success: true }
-}