Merge pull request '결제 전환 목표 아키텍처 정의 (2/2)' (#2) from forge/iss-c7e589ce4a08-attempt-1 into main

.forge/iss-c7e589ce4a08-attempt-1.md

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+# iss-c7e589ce4a08-attempt-1
+
+Forge 이슈 작업 브랜치 `forge/iss-c7e589ce4a08-attempt-1`.

docs/architecture/transaction-sequence.md

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+sequenceDiagram
+    participant User
+    participant Payment Gateway
+    participant Database
+    participant Service
+
+    User->>Service: Initiate Payment
+    Service->>Payment Gateway: Send Payment Request
+    alt Successful Payment
+        Payment Gateway-->>Service: Confirm Payment
+        Service->>Database: Record Payment
+        Database-->>Service: Payment Recorded
+        Service-->>User: Payment Success
+    else External Payment Failed
+        Payment Gateway-->>Service: Payment Failed
+        Service-->>User: Payment Failed
+    else Database Failure
+        Service->>Database: Attempt to Record Payment
+        Database-->>Service: Database Error
+        Service-->>User: Unable to process payment
+    end
+    Service->>Database: Commit
+    alt Rollback Required
+        Database->>Database: Rollback Changes
+    end