MS-245/feat(products): Add optimized product cards search endpoint with dynamic filter counts

Summary

This MR introduces a new optimized endpoint for searching and displaying product cards on the frontend, with dynamic filter counts for better UX.

Motivation

The existing product listing endpoints return heavy DTOs with unnecessary data for card display. This creates performance issues on product listing pages. Additionally, the frontend needs real-time counts for "In Stock" and "On Sale" filters.

Changes

New DTOs

DTO	Purpose
ProductCardDTO	Lightweight DTO with only card-relevant fields
ProductCardSearchCriteria	Encapsulates all search/filter parameters
ProductCardPageResponse	Paginated response with filter counts
FilterCountsDTO	Record containing inStock and onSale counts

New Endpoint

GET /api/stock/products/cards/search

Query Parameters:

• categorySlug (optional): Filter by category slug (includes subcategories)
• minPrice / maxPrice (optional): Price range filter
• onSale (optional): Filter products on sale (promoPrice > 0)
• inStock (optional): Filter available products
• keyword (optional): Search in name and description
• page (default: 0): Page number
• size (default: 12): Page size
• sortField (default: createdAt): Sort field
• sortDirection (default: desc): Sort direction

Response includes:

• Paginated list of ProductCardDTO
• filterCounts.inStockCount: Number of available products
• filterCounts.onSaleCount: Number of products on sale

Performance Optimizations

• Direct DTO projection in JPQL queries (no entity mapping overhead)
• Single query for products + separate count queries for filters
• Efficient category hierarchy handling

Breaking Changes

• CategoryService.getRelatedProducts() now returns List<ProductCardDTO> instead of List<CustomProductRequest>
• ProductService.getProductByCategorySlug() now returns Page<ProductCardDTO> instead of Page<CustomProductRequest>

Closes MS-245