refactor(email): abstract email system with proper architecture

- Create service layer for email generation (PaymentMethodService, OrderTransformerService, EmailBuilderService)
- Extract all hardcoded values to configuration files
- Add proper TypeScript types for all email data structures
- Update GraphQL subscription to include transactions and events
- Refactor order-created, order-cancelled templates to use new architecture
- Payment method now dynamically detected from order transactions

This makes the email system:
- Testable: Each service can be unit tested independently
- Configurable: All values in config.ts, no hardcoding
- Extensible: Easy to add new languages or payment methods
- Maintainable: Clear separation of concerns

src/pages/api/webhooks/order-cancelled.ts

@@ -4,7 +4,7 @@ import {
   OrderCancelledWebhookPayloadFragment,
 } from "@/generated/graphql";
 import { saleorApp } from "@/saleor-app";
-import { getOrderCancelledEmails } from "@/lib/email-templates";
+import { getOrderCancelledEmails } from "@/lib/email-templates/order-cancelled";
 import { Resend } from "resend";
 
 const resend = new Resend(process.env.RESEND_API_KEY);