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

stores/product-workspace/store.ts

@@ -557,10 +557,6 @@ export const useProductWorkspaceStore = create<ProductWorkspaceStore>()(
           case 'pbi-order':
             // store-call passes new order via separate set, snapshot is prevPbiIds
             break
-          case 'story-order':
-            break
-          case 'task-order':
-            break
           case 'entity-patch':
             break
         }
@@ -577,12 +573,6 @@ export const useProductWorkspaceStore = create<ProductWorkspaceStore>()(
           case 'pbi-order':
             s.relations.pbiIds = [...mutation.prevPbiIds]
             break
-          case 'story-order':
-            s.relations.storyIdsByPbi[mutation.pbiId] = [...mutation.prevStoryIds]
-            break
-          case 'task-order':
-            s.relations.taskIdsByStory[mutation.storyId] = [...mutation.prevTaskIds]
-            break
           case 'entity-patch': {
             const { entity, id, prev } = mutation
             if (prev) {

stores/product-workspace/types.ts

@@ -109,18 +109,6 @@ export interface OptimisticPbiOrderMutation {
   prevPbiIds: string[]
 }
 
-export interface OptimisticStoryOrderMutation {
-  kind: 'story-order'
-  pbiId: string
-  prevStoryIds: string[]
-}
-
-export interface OptimisticTaskOrderMutation {
-  kind: 'task-order'
-  storyId: string
-  prevTaskIds: string[]
-}
-
 export interface OptimisticEntityPatchMutation {
   kind: 'entity-patch'
   entity: 'pbi' | 'story' | 'task'
@@ -130,8 +118,6 @@ export interface OptimisticEntityPatchMutation {
 
 export type OptimisticMutation =
   | OptimisticPbiOrderMutation
-  | OptimisticStoryOrderMutation
-  | OptimisticTaskOrderMutation
   | OptimisticEntityPatchMutation
 
 export interface PendingOptimisticMutation {

stores/sprint-workspace/store.ts

@@ -519,12 +519,6 @@ export const useSprintWorkspaceStore = create<SprintWorkspaceStore>()(
       const { mutation } = pending
       set((s) => {
         switch (mutation.kind) {
-          case 'sprint-story-order':
-            s.relations.storyIdsBySprint[mutation.sprintId] = [...mutation.prevStoryIds]
-            break
-          case 'sprint-task-order':
-            s.relations.taskIdsByStory[mutation.storyId] = [...mutation.prevTaskIds]
-            break
           case 'entity-patch': {
             const { entity, id, prev } = mutation
             if (prev) {

stores/sprint-workspace/types.ts

@@ -122,18 +122,6 @@ export type ResyncReason =
 
 export type RealtimeStatus = 'connecting' | 'open' | 'disconnected'
 
-export interface OptimisticSprintStoryOrderMutation {
-  kind: 'sprint-story-order'
-  sprintId: string
-  prevStoryIds: string[]
-}
-
-export interface OptimisticSprintTaskOrderMutation {
-  kind: 'sprint-task-order'
-  storyId: string
-  prevTaskIds: string[]
-}
-
 export interface OptimisticEntityPatchMutation {
   kind: 'entity-patch'
   entity: 'sprint' | 'story' | 'task'
@@ -147,8 +135,6 @@ export interface OptimisticEntityPatchMutation {
 }
 
 export type OptimisticMutation =
-  | OptimisticSprintStoryOrderMutation
-  | OptimisticSprintTaskOrderMutation
   | OptimisticEntityPatchMutation
 
 export interface PendingOptimisticMutation {