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

app/api/stories/[id]/tasks/reorder/route.ts

@@ -1,56 +0,0 @@
-import { authenticateApiRequest } from '@/lib/api-auth'
-import { prisma } from '@/lib/prisma'
-import { productAccessFilter } from '@/lib/product-access'
-import { z } from 'zod'
-
-const bodySchema = z.object({
-  task_ids: z.array(z.string()).min(1),
-})
-
-export async function PATCH(
-  request: Request,
-  { params }: { params: Promise<{ id: string }> }
-) {
-  const auth = await authenticateApiRequest(request)
-  if ('error' in auth) {
-    return Response.json({ error: auth.error }, { status: auth.status })
-  }
-  if (auth.isDemo) {
-    return Response.json({ error: 'Niet beschikbaar in demo-modus' }, { status: 403 })
-  }
-
-  const { id: storyId } = await params
-
-  let body: unknown
-  try {
-    body = await request.json()
-  } catch {
-    return Response.json({ error: 'Malformed JSON' }, { status: 400 })
-  }
-  const parsed = bodySchema.safeParse(body)
-  if (!parsed.success) {
-    return Response.json({ error: parsed.error.flatten() }, { status: 422 })
-  }
-
-  const story = await prisma.story.findFirst({
-    where: { id: storyId, product: productAccessFilter(auth.userId) },
-    include: { tasks: { select: { id: true } } },
-  })
-  if (!story) {
-    return Response.json({ error: 'Story niet gevonden' }, { status: 404 })
-  }
-
-  const storyTaskIds = new Set(story.tasks.map(t => t.id))
-  const invalidId = parsed.data.task_ids.find(id => !storyTaskIds.has(id))
-  if (invalidId) {
-    return Response.json({ error: `Ongeldig task_id: ${invalidId}` }, { status: 422 })
-  }
-
-  await prisma.$transaction(
-    parsed.data.task_ids.map((id, i) =>
-      prisma.task.update({ where: { id }, data: { sort_order: i + 1.0 } })
-    )
-  )
-
-  return Response.json({ success: true })
-}