feat(backlog): add BacklogStore Zustand store with applyChange reducer

State: pbis, storiesByPbi, tasksByStory. setInitialData for server
hydration; applyChange(entity, op, data) handles I/U/D for SSE events.

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

stores/backlog-store.ts

@@ -0,0 +1,139 @@
+import { create } from 'zustand'
+import type { PbiStatusApi } from '@/lib/task-status'
+
+export interface BacklogPbi {
+  id: string
+  code: string | null
+  title: string
+  priority: number
+  description?: string | null
+  created_at: Date
+  status: PbiStatusApi
+}
+
+export interface BacklogStory {
+  id: string
+  code: string | null
+  title: string
+  description: string | null
+  acceptance_criteria: string | null
+  priority: number
+  status: string
+  pbi_id: string
+  created_at: Date
+}
+
+export interface BacklogTask {
+  id: string
+  title: string
+  description: string | null
+  priority: number
+  status: string
+  sort_order: number
+  story_id: string
+  created_at: Date
+}
+
+type Entity = 'pbi' | 'story' | 'task'
+type Op = 'I' | 'U' | 'D'
+
+interface InitialData {
+  pbis: BacklogPbi[]
+  storiesByPbi: Record<string, BacklogStory[]>
+  tasksByStory: Record<string, BacklogTask[]>
+}
+
+interface BacklogStore extends InitialData {
+  setInitialData: (data: InitialData) => void
+  applyChange: (entity: Entity, op: Op, data: Record<string, unknown>) => void
+}
+
+export const useBacklogStore = create<BacklogStore>((set) => ({
+  pbis: [],
+  storiesByPbi: {},
+  tasksByStory: {},
+
+  setInitialData: (data) => set(data),
+
+  applyChange: (entity, op, data) =>
+    set((state) => {
+      if (entity === 'pbi') {
+        const id = data.id as string
+        if (op === 'D') {
+          return { pbis: state.pbis.filter((p) => p.id !== id) }
+        }
+        if (op === 'U') {
+          return {
+            pbis: state.pbis.map((p) =>
+              p.id === id ? { ...p, ...(data as Partial<BacklogPbi>) } : p
+            ),
+          }
+        }
+        // I
+        return { pbis: [...state.pbis, data as unknown as BacklogPbi] }
+      }
+
+      if (entity === 'story') {
+        const id = data.id as string
+        if (op === 'D') {
+          const storiesByPbi = { ...state.storiesByPbi }
+          for (const pbiId of Object.keys(storiesByPbi)) {
+            storiesByPbi[pbiId] = storiesByPbi[pbiId].filter((s) => s.id !== id)
+          }
+          return { storiesByPbi }
+        }
+        if (op === 'U') {
+          const storiesByPbi = { ...state.storiesByPbi }
+          for (const pbiId of Object.keys(storiesByPbi)) {
+            const idx = storiesByPbi[pbiId].findIndex((s) => s.id === id)
+            if (idx !== -1) {
+              storiesByPbi[pbiId] = storiesByPbi[pbiId].map((s) =>
+                s.id === id ? { ...s, ...(data as Partial<BacklogStory>) } : s
+              )
+              break
+            }
+          }
+          return { storiesByPbi }
+        }
+        // I
+        const pbiId = data.pbi_id as string
+        return {
+          storiesByPbi: {
+            ...state.storiesByPbi,
+            [pbiId]: [...(state.storiesByPbi[pbiId] ?? []), data as unknown as BacklogStory],
+          },
+        }
+      }
+
+      // task
+      const id = data.id as string
+      if (op === 'D') {
+        const tasksByStory = { ...state.tasksByStory }
+        for (const storyId of Object.keys(tasksByStory)) {
+          tasksByStory[storyId] = tasksByStory[storyId].filter((t) => t.id !== id)
+        }
+        return { tasksByStory }
+      }
+      if (op === 'U') {
+        const tasksByStory = { ...state.tasksByStory }
+        for (const storyId of Object.keys(tasksByStory)) {
+          const idx = tasksByStory[storyId].findIndex((t) => t.id === id)
+          if (idx !== -1) {
+            tasksByStory[storyId] = tasksByStory[storyId].map((t) =>
+              t.id === id ? { ...t, ...(data as Partial<BacklogTask>) } : t
+            )
+            break
+          }
+        }
+        return { tasksByStory }
+      }
+      // I
+      const storyId = data.story_id as string
+      return {
+        tasksByStory: {
+          ...state.tasksByStory,
+          [storyId]: [...(state.tasksByStory[storyId] ?? []), data as unknown as BacklogTask],
+        },
+      }
+    }),
+}))