[data-model] Tenant/source order identity migration ADR

[OziinG]

Design and implement additive tenant/source order identity migration.

Current Shopify-shaped seams:

• Shop.shopDomain
• Shop.shopifyShopGid
• Order.shopifyOrderGid
• .myshopify.com validators
• driver/mobile contracts exposing shopDomain

Decision direction:

• Add platform-neutral identity first.
• Keep legacy Shopify fields until compatibility tests pass.
• Introduce sourcePlatform, sourceOrderId, sourceOrderNumber, sourceUpdatedAt or equivalent mapping.
• No destructive Prisma/table/column rename in the first pass.