docs/architecture/transaction-sequence.md

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+%%%{init: {'themeVariables': {'sequenceDiagramTheme': 'default'}}}%%%
+
+sequenceDiagram
+    participant User
+    participant PaymentGateway
+    participant Database
+
+    User->>PaymentGateway: Initiate Payment
+    alt Successful Payment
+        PaymentGateway->>Database: Commit Transaction
+        Database-->>PaymentGateway: Confirm Commit
+        PaymentGateway-->>User: Payment Success
+    else External Failure
+        PaymentGateway-->>User: Payment Failure
+    else DB Failure
+        PaymentGateway->>Database: Rollback Transaction
+        Database-->>PaymentGateway: Confirm Rollback
+        PaymentGateway-->>User: Payment Rollback
+    end