diff --git a/.forge/iss-5a912ed79baf-attempt-2.md b/.forge/iss-5a912ed79baf-attempt-2.md
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+# iss-5a912ed79baf-attempt-2
+
+Forge 이슈 작업 브랜치 `forge/iss-5a912ed79baf-attempt-2`.
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 Database
+
+    User->>PaymentGateway: Initiate payment
+    PaymentGateway->>User: Payment status (success)
+    User->>Database: Save transaction
+    Database-->>User: Transaction saved
+
+    %% External failure scenario
+    User->>PaymentGateway: Initiate payment
+    PaymentGateway-->>User: Payment status (failure)
+    User->>Database: Log failure reason
+
+    %% Database failure scenario
+    User->>PaymentGateway: Initiate payment
+    PaymentGateway->>User: Payment status (success)
+    User->>Database: Save transaction
+    Database-->>User: Transaction failed (DB error)
+    Note right of User: Rollback transaction
+```
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