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

__tests__/stores/sprint-workspace/store.test.ts

@@ -852,56 +852,6 @@ describe('restore-hint flow — chain triggert na ensure*Loaded', () => {
 // ─────────────────────────────────────────────────────────────────────────
 
 describe('optimistic mutations', () => {
-  it('rollback herstelt vorige sprint-story-order', () => {
-    useSprintWorkspaceStore.getState().hydrateSnapshot(
-      snapshotWith(
-        makeSprint({ id: 'sp-1', product_id: 'prod-1' }),
-        [
-          makeStory({ id: 'a', pbi_id: 'p', sprint_id: 'sp-1', sort_order: 1 }),
-          makeStory({ id: 'b', pbi_id: 'p', sprint_id: 'sp-1', sort_order: 2 }),
-        ],
-      ),
-    )
-    const prevOrder = [
-      ...useSprintWorkspaceStore.getState().relations.storyIdsBySprint['sp-1'],
-    ]
-
-    const id = useSprintWorkspaceStore.getState().applyOptimisticMutation({
-      kind: 'sprint-story-order',
-      sprintId: 'sp-1',
-      prevStoryIds: prevOrder,
-    })
-    useSprintWorkspaceStore.setState((s) => {
-      s.relations.storyIdsBySprint['sp-1'] = ['b', 'a']
-    })
-
-    useSprintWorkspaceStore.getState().rollbackMutation(id)
-    expect(useSprintWorkspaceStore.getState().relations.storyIdsBySprint['sp-1']).toEqual(
-      prevOrder,
-    )
-    expect(useSprintWorkspaceStore.getState().pendingMutations[id]).toBeUndefined()
-  })
-
-  it('settle ruimt pending op zonder state te wijzigen', () => {
-    useSprintWorkspaceStore.getState().hydrateSnapshot(
-      snapshotWith(makeSprint({ id: 'sp-1', product_id: 'prod-1' }), [
-        makeStory({ id: 'a', pbi_id: 'p', sprint_id: 'sp-1' }),
-      ]),
-    )
-    const id = useSprintWorkspaceStore.getState().applyOptimisticMutation({
-      kind: 'sprint-story-order',
-      sprintId: 'sp-1',
-      prevStoryIds: ['a'],
-    })
-    expect(useSprintWorkspaceStore.getState().pendingMutations[id]).toBeDefined()
-
-    useSprintWorkspaceStore.getState().settleMutation(id)
-    expect(useSprintWorkspaceStore.getState().pendingMutations[id]).toBeUndefined()
-    expect(useSprintWorkspaceStore.getState().relations.storyIdsBySprint['sp-1']).toEqual([
-      'a',
-    ])
-  })
-
   it('SSE-echo van een al-bestaande sprint is idempotent', () => {
     useSprintWorkspaceStore.setState((s) => {
       s.context.activeProduct = { id: 'prod-1', name: 'P' }