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

prisma/schema.prisma

@@ -29,4 +29,12 @@ model Item {
   updatedAt   DateTime  @updatedAt
   listId      String
   list        List      @relation(fields: [listId], references: [id], onDelete: Cascade)
+
+  @@index([listId])
+  @@index([completed])
+  @@index([priority])
+  @@index([dueDate])
+  @@index([createdAt])
+  @@index([listId, completed])
+  @@index([listId, priority])
 }