결제 전환 목표 아키텍처 정의 (2/2) (iss-2499ff1ad79a)

docs/architecture/transaction-sequence.md

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+```mermaid
+sequenceDiagram
+    participant User
+    participant PaymentGateway
+    participant Application
+    participant Database
+    User->>Application: Initiate Payment
+    Application->>PaymentGateway: Process Payment
+    alt Successful Payment
+        PaymentGateway-->>Application: Payment Approved
+        Application->>Database: Commit Transaction
+        Database-->>Application: Transaction Committed
+        Application-->>User: Payment Successful
+    else External Failure
+        PaymentGateway-->>Application: Payment Error
+        Application-->>User: Payment Failed
+    else DB Failure
+        Application->>Database: Attempt Transaction
+        Database-->>Application: Transaction Error
+        Application->>Database: Rollback Transaction
+        Database-->>Application: Transaction Rolled Back
+        Application-->>User: Payment Failed
+    end
+```