Add multi-catalog architecture and language persistence fix

Phase 1: Database Schema & Migration
- Added Catalog and CatalogCustomer models to Prisma schema
- Updated Category, Product, BannerSlide with catalogId foreign keys
- Created seed script for default catalog migration
- Added test Docker Compose configuration
- Created migration scripts for test and production

Bug Fixes:
- Fixed language persistence across logout/login sessions
- Language preference now preserved in localStorage

Testing Infrastructure:
- Added comprehensive testing guide
- Created isolated test environment (different ports/containers)
- Test environment safe to run alongside production

Files Added:
- backend/prisma/seed-default-catalog.ts
- backend/docker-compose.test.yml
- backend/.env.test
- backend/test-migration.sh
- backend/migrate-multi-catalog.sh
- TESTING_GUIDE.md
