feat: implement real-time list synchronization

- Add useRealtimeSync hook for polling server updates every 3 seconds
- Create API endpoint to check last updated timestamp for lists
- Integrate real-time sync into ListDetailView component
- Automatically refresh when other users make changes to shared lists
- Poll on interval and when page becomes visible again

hooks/useRealtimeSync.ts

@@ -0,0 +1,106 @@
+'use client'
+
+import { useEffect, useRef, useCallback, useState } from 'react'
+import { useRouter } from 'next/navigation'
+
+interface UseRealtimeSyncOptions {
+  listId: string
+  enabled?: boolean
+  pollInterval?: number
+  onUpdate?: () => void
+}
+
+/**
+ * Hook to enable real-time synchronization of list data
+ * Polls the server for updates and refreshes the page when changes are detected
+ */
+export function useRealtimeSync({
+  listId,
+  enabled = true,
+  pollInterval = 3000, // Poll every 3 seconds
+  onUpdate
+}: UseRealtimeSyncOptions) {
+  const router = useRouter()
+  const [lastUpdated, setLastUpdated] = useState<number>(Date.now())
+  const intervalRef = useRef<NodeJS.Timeout | null>(null)
+  const isPollingRef = useRef(false)
+
+  const checkForUpdates = useCallback(async () => {
+    // Prevent concurrent polling
+    if (isPollingRef.current) return
+    
+    try {
+      isPollingRef.current = true
+      
+      // Fetch the latest update timestamp for the list
+      const response = await fetch(`/api/lists/${listId}/last-updated`, {
+        method: 'GET',
+        headers: {
+          'Cache-Control': 'no-cache',
+        },
+      })
+
+      if (response.ok) {
+        const data = await response.json()
+        const serverTimestamp = new Date(data.lastUpdated).getTime()
+        
+        // If server timestamp is newer than our last known update, refresh
+        if (serverTimestamp > lastUpdated) {
+          setLastUpdated(serverTimestamp)
+          
+          // Refresh the current route to get latest data
+          router.refresh()
+          
+          // Call optional callback
+          if (onUpdate) {
+            onUpdate()
+          }
+        }
+      }
+    } catch (error) {
+      console.error('Error checking for updates:', error)
+    } finally {
+      isPollingRef.current = false
+    }
+  }, [listId, lastUpdated, router, onUpdate])
+
+  useEffect(() => {
+    if (!enabled) {
+      if (intervalRef.current) {
+        clearInterval(intervalRef.current)
+        intervalRef.current = null
+      }
+      return
+    }
+
+    // Start polling
+    intervalRef.current = setInterval(checkForUpdates, pollInterval)
+
+    // Cleanup on unmount
+    return () => {
+      if (intervalRef.current) {
+        clearInterval(intervalRef.current)
+        intervalRef.current = null
+      }
+    }
+  }, [enabled, pollInterval, checkForUpdates])
+
+  // Also check for updates when the page becomes visible again
+  useEffect(() => {
+    const handleVisibilityChange = () => {
+      if (document.visibilityState === 'visible' && enabled) {
+        checkForUpdates()
+      }
+    }
+
+    document.addEventListener('visibilitychange', handleVisibilityChange)
+    return () => {
+      document.removeEventListener('visibilitychange', handleVisibilityChange)
+    }
+  }, [enabled, checkForUpdates])
+
+  return {
+    lastUpdated,
+    checkForUpdates,
+  }
+}