feat(design) created an ER diagram

design.md

@@ -63,6 +63,41 @@ flowchart LR
 - Example records or schemas (link to files or include concise snippets).
 - Users, Transactions, Transaction_Categories, User_settings
 
+```mermaid
+erDiagram
+    USERS {
+        int user_id PK
+        string name
+        string email
+    }
+
+    TRANSACTIONS {
+        int transaction_id PK
+        int user_id FK
+        int category_id FK
+        decimal amount
+        date transaction_date
+    }
+
+    TRANSACTION_CATEGORIES {
+        int category_id PK
+        string name
+        string type
+        string color
+    }
+
+    USER_SETTINGS {
+        int setting_id PK
+        int user_id FK
+        string setting_key
+        string setting_value
+    }
+
+    USERS ||--o{ TRANSACTIONS : "makes"
+    USERS ||--o{ USER_SETTINGS : "has"
+    TRANSACTION_CATEGORIES ||--o{ TRANSACTIONS : "categorizes"
+```
+
 ### 2.3 APIs (REST/gRPC/GraphQL)
 
 - Swagger