feat: implement multi-language translation support for products
- Add database migrations for translation tables and digital product fields
- Create MarketplaceProductTranslation model for localized content
- Create ProductTranslation model for vendor translation overrides
- Add product type enum (physical, digital, service, subscription)
- Add digital delivery method and platform fields
- Add numeric price fields (price_numeric, sale_price_numeric)
- Implement language fallback pattern (requested -> 'en' -> None)
- Add helper methods: get_title(), get_description(), get_translation()
- Add vendor override pattern with effective_* properties
- Move title/description from products to translations table
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>