Proto Definitions: Add discount code fields to checkout messages

[rishikeshradhakrishnan]

Parent Issue

Part of #96 — Add promotional discount code feature to checkout flow

Service

Proto / gRPC definitions (shared across services)

Scope

PlaceOrderRequest Update

Add a discount_code field to PlaceOrderRequest so the frontend can pass the user's coupon code to the Checkout service:

message PlaceOrderRequest {
  string user_id = 1;
  string user_currency = 2;
  Address address = 3;
  string email = 5;
  CreditCardInfo credit_card = 6;
  string discount_code = 7;  // NEW: promotional discount code
}

OrderResult Update

Add discount details to OrderResult so the frontend and email service can display them:

message OrderResult {
  string order_id = 1;
  string shipping_tracking_id = 2;
  Money shipping_cost = 3;
  Address shipping_address = 4;
  repeated OrderItem items = 5;
  DiscountDetail discount = 6;  // NEW: applied discount info
}

// NEW message
message DiscountDetail {
  string code = 1;           // e.g. "SAVE10"
  string description = 2;    // e.g. "10% off entire order"
  Money discount_amount = 3; // amount saved
}

Code Generation

• Regenerate gRPC stubs for all affected languages:
  • Go (checkout service)
  • TypeScript (frontend)
  • Node.js (payment service, if proto is used directly)
• Ensure backward compatibility — new fields are optional so existing clients continue to work

Acceptance Criteria

• PlaceOrderRequest includes discount_code field
• OrderResult includes DiscountDetail message
• DiscountDetail message defined with code, description, and discount_amount
• gRPC stubs regenerated for Go, TypeScript, and Node.js
• Existing services compile and pass tests without changes (backward compatible)
• Proto linting passes (if configured)