Merge pull request '결제 전환 목표 아키텍처 정의 (2/2)' (#1) from forge/iss-81c03d204ddd-attempt-1-run-c46c886cd339 into main

.forge/iss-81c03d204ddd-attempt-1-run-c46c886cd339.md

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+# iss-81c03d204ddd-attempt-1-run-c46c886cd339
+
+Forge 이슈 작업 브랜치 `forge/iss-81c03d204ddd-attempt-1-run-c46c886cd339`.

docs/architecture/transaction-sequence.md

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+```mermaid
+sequenceDiagram
+    participant User
+    participant PaymentGateway
+    participant PaymentService
+    participant Database
+
+    User->>+PaymentService: Start Payment
+    PaymentService->>+PaymentGateway: Process Payment
+    PaymentGateway-->>-PaymentService: Payment Success
+    PaymentService->>Database: Record Payment
+    Database-->>-PaymentService: Payment Recorded
+    PaymentService-->>-User: Payment Confirmed
+
+    %% External Failure
+    User->>+PaymentService: Start Payment
+    PaymentService->>+PaymentGateway: Process Payment
+    PaymentGateway-->>-PaymentService: Payment Failed
+    PaymentService-->>-User: Payment Failed
+
+    %% DB Failure
+    User->>+PaymentService: Start Payment
+    PaymentService->>+PaymentGateway: Process Payment
+    PaymentGateway-->>-PaymentService: Payment Success
+    PaymentService->>Database: Record Payment
+    Database-->>-PaymentService: DB Failure
+    PaymentService-->>-User: Payment Processing Error
+
+    %% Commit/Rollback
+    Note over PaymentService: Commit if successful
+    Note over PaymentService: Rollback if there's an error
+```