V47: Shop Localized Fields

Converts Shop module from separate translations JSONB to localized fields approach.

Changes

• Product fields (title, slug, description, body_html, seo_title, seo_description) change from VARCHAR/TEXT to JSONB maps
• Category fields (name, slug, description) change from VARCHAR/TEXT to JSONB maps
• Removes translations column from both tables
• Each field stores language → value map: %{"en" => "Product", "ru" => "Продукт"}

Migration Strategy

1. Add temporary _new columns as JSONB
2. Migrate canonical data with default language key from settings
3. Merge existing translations into new fields
4. Drop old columns, rename _new to original names
5. Create GIN indexes for slug lookups

Rollback

Extracts default language values back to string columns and rebuilds the translations map from non-default language data.