feat: implement performance optimizations

- Add database indexes for frequently queried fields (listId, completed, priority, dueDate, createdAt)
- Add compound indexes for common query patterns (listId+completed, listId+priority)
- Implement pagination support in getItemsByFilter with configurable page size (default 50)
- Optimize components with React.memo (ItemCard, ListCard, FilterPanel, SearchBar, SearchResults, EmptyState)
- Configure Next.js for better bundle optimization:
  - Enable optimizePackageImports for lucide-react
  - Enable SWC minification
  - Remove console logs in production (except errors/warnings)
  - Optimize images with AVIF/WebP formats
  - Enable compression
  - Disable production source maps for smaller bundles
- Loading states already implemented to prevent layout shift

Task 18 complete - all performance optimizations implemented

components/ItemCard.tsx

@@ -1,5 +1,6 @@
 'use client'
 
+import { memo } from 'react'
 import Link from 'next/link'
 import { Calendar, Tag, AlertCircle } from 'lucide-react'
 import ToggleItemButton from './ToggleItemButton'
@@ -19,7 +20,7 @@ interface ItemCardProps {
   listId: string
 }
 
-export default function ItemCard({ item, listId }: ItemCardProps) {
+function ItemCard({ item, listId }: ItemCardProps) {
   const tags = Array.isArray(item.tags) ? item.tags : []
   const isOverdue = item.dueDate && new Date(item.dueDate) < new Date() && !item.completed
 
@@ -104,3 +105,5 @@ export default function ItemCard({ item, listId }: ItemCardProps) {
     </div>
   )
 }
+
+export default memo(ItemCard)