Merge pull request '결제 전환 목표 아키텍처 정의 (2/2)' (#2) from forge/iss-1b3092823a38-attempt-1 into main

docs/architecture/transaction-sequence.md

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+sequenceDiagram
+    participant User
+    participant Payment Service
+    participant DB
+    participant External Gateway
+
+    User->>Payment Service: Initiate Payment
+    activate Payment Service
+    Payment Service->>DB: Begin Transaction
+    activate DB
+    DB-->>Payment Service: Transaction Started
+    deactivate DB
+    Payment Service->>External Gateway: Request Payment Authorization
+    activate External Gateway
+
+    alt Successful Authorization
+        External Gateway-->>Payment Service: Authorization Success
+        Payment Service->>DB: Commit Transaction
+        activate DB
+        DB-->>Payment Service: Transaction Committed
+        deactivate DB
+        Payment Service-->>User: Payment Successful
+    else Authorization Failed
+        External Gateway-->>Payment Service: Authorization Failure
+        Payment Service->>DB: Rollback Transaction
+        activate DB
+        DB-->>Payment Service: Transaction Rolled Back
+        deactivate DB
+        Payment Service-->>User: Payment Failed
+    end
+
+autonumber
+
+