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

graphql/fragments/order-created.graphql

@@ -86,5 +86,14 @@ fragment OrderCreatedWebhookPayload on OrderCreated {
       key
       value
     }
+    transactions {
+      id
+      message
+      externalUrl
+      events {
+        message
+        type
+      }
+    }
   }
 }