diff --git a/.forge/iss-155aa95ea6df-attempt-1.md b/.forge/iss-155aa95ea6df-attempt-1.md
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+# iss-155aa95ea6df-attempt-1
+
+Forge 이슈 작업 브랜치 `forge/iss-155aa95ea6df-attempt-1`.
diff --git a/docs/architecture/transaction-sequence.md b/docs/architecture/transaction-sequence.md
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+```mermaid
+sequenceDiagram
+    participant User
+    participant PaymentGateway
+    participant Bank
+
+    %% Normal flow
+    User ->> PaymentGateway: Request Payment
+    PaymentGateway ->> Bank: Process Payment
+    Bank -->> PaymentGateway: Payment Approved
+    PaymentGateway -->> User: Payment Confirmation
+
+    %% External failure branch
+    User ->> PaymentGateway: Request Payment
+    PaymentGateway ->> Bank: Process Payment
+    Bank -->> PaymentGateway: Payment Declined
+    PaymentGateway -->> User: Payment Declined Message
+
+    %% DB failure branch
+    User ->> PaymentGateway: Request Payment
+    PaymentGateway ->> Bank: Process Payment
+    Bank -->> PaymentGateway: DB Error
+    PaymentGateway -->> User: Payment Failed Due to Error
+    Note over PaymentGateway: Rollback transaction if needed
+```
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