fix: return empty array instead of null for list endpoints to prevent FE crashes

Optimize database query logs, GetProduct, GetMe and allow clearing ImageURL

perf: completely avoid SELECT * in DeleteProduct, GetCategory, UpdateCategory, and DeleteCategory handlers by using direct database updates and specific select counts

perf: completely avoid SELECT * in CreateProduct and UpdateProduct by selecting Category directly instead of First product load

feat: auto-generate unique SKU when blank during product creation to match Python FastAPI schema validation behavior

fix: normalize empty string SKU to nil/NULL to prevent unique key constraint violations on duplicate empty SKUs

perf: optimize ListProducts, ListCategories, and ListOrders GORM queries with select-specific-columns and custom nested preload column selects

perf: optimize ListProducts using database-level pagination, search, and branch filtering

feat: migrate warungpos backend to Go with Tripay