feat(PBI-74): sprint hydratie + realtime SSE (Story 9 / T-880)

- app/api/realtime/sprint/route.ts: SSE-stream LISTEN/NOTIFY op
  scrum4me_changes, filter entity ∈ {sprint, story, task} per product_id;
  ready-event, heartbeat 25s, hard-close 240s
- lib/realtime/use-sprint-realtime.ts: client-hook met backoff-reconnect;
  ready-cycle telt; geen close op hidden; setRealtimeStatus
- lib/realtime/use-sprint-workspace-resync.ts: visibility + online triggers
  resyncActiveScopes('visible' | 'reconnect')
- components/sprint/sprint-hydration-wrapper.tsx: hydrateSnapshot via
  useEffect met fingerprint-check; mount realtime + resync
- app/(app)/products/[id]/sprint/[sprintId]/page.tsx: wrap SprintBoardClient
  in SprintHydrationWrapper; bouw SprintWorkspaceTask-shape voor
  tasksByStoryWorkspace en SprintHydrationData voor de wrapper

Schaduw-fase: useSprintStore blijft parallel werken in board components
totdat T-881 die migreert en T-883 de oude store opruimt.

components/sprint/sprint-hydration-wrapper.tsx

@@ -0,0 +1,86 @@
+'use client'
+
+// PBI-74 / Story 9 / T-880: Sprint workspace hydration wrapper.
+//
+// Server-component (sprint page) fetcht initial sprint snapshot; deze wrapper
+// hydreert useSprintWorkspaceStore op client-mount, mount de SSE-hook en de
+// resync-laag. Tijdens T-880 (schaduw-fase) blijft useSprintStore parallel
+// werken in sprint-board components — beide stores naast elkaar tot T-881
+// componenten omzet en T-883 de oude store opruimt.
+
+import { useEffect, useRef } from 'react'
+import { useSprintRealtime } from '@/lib/realtime/use-sprint-realtime'
+import { useSprintWorkspaceResync } from '@/lib/realtime/use-sprint-workspace-resync'
+import { useSprintWorkspaceStore } from '@/stores/sprint-workspace/store'
+import type {
+  SprintWorkspaceSnapshot,
+  SprintWorkspaceSprint,
+  SprintWorkspaceStory,
+  SprintWorkspaceTask,
+} from '@/stores/sprint-workspace/types'
+
+export interface SprintHydrationData {
+  sprint: SprintWorkspaceSprint
+  stories: SprintWorkspaceStory[]
+  tasksByStory: Record<string, SprintWorkspaceTask[]>
+}
+
+interface SprintHydrationWrapperProps {
+  initialData: SprintHydrationData
+  productId: string
+  productName?: string
+  children: React.ReactNode
+}
+
+function fingerprint(data: SprintHydrationData): string {
+  const sprintPart = `${data.sprint.id}:${data.sprint.status}`
+  const storyPart = data.stories
+    .map((s) => `${s.id}:${s.status}:${s.sprint_id ?? 'null'}:${s.sort_order}`)
+    .join(',')
+  const taskPart = Object.entries(data.tasksByStory)
+    .flatMap(([, list]) => list.map((t) => `${t.id}:${t.status}:${t.sort_order}`))
+    .join(',')
+  return `${sprintPart}|${storyPart}|${taskPart}`
+}
+
+function toWorkspaceSnapshot(
+  data: SprintHydrationData,
+  productId: string,
+  productName: string | undefined,
+): SprintWorkspaceSnapshot {
+  return {
+    product: { id: productId, name: productName ?? '' },
+    sprint: data.sprint,
+    stories: data.stories,
+    tasksByStory: data.tasksByStory,
+  }
+}
+
+export function SprintHydrationWrapper({
+  initialData,
+  productId,
+  productName,
+  children,
+}: SprintHydrationWrapperProps) {
+  const lastFingerprint = useRef<string>('')
+
+  useEffect(() => {
+    const fp = fingerprint(initialData)
+    if (fp !== lastFingerprint.current) {
+      lastFingerprint.current = fp
+      useSprintWorkspaceStore
+        .getState()
+        .hydrateSnapshot(toWorkspaceSnapshot(initialData, productId, productName))
+      // T-880 schaduw-fase: zet activeSprintId zodat selectors meteen werken
+      useSprintWorkspaceStore.setState((s) => {
+        s.context.activeSprintId = initialData.sprint.id
+        s.context.activeProduct = { id: productId, name: productName ?? '' }
+      })
+    }
+  }, [initialData, productId, productName])
+
+  useSprintRealtime(productId)
+  useSprintWorkspaceResync()
+
+  return <>{children}</>
+}