docs: erd 추가

Author: yeonjiyeon

docs/week2/04-erd.md

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+## ERD
+
+---
+```mermaid
+erDiagram
+    USER ||--o{ POINT : holds
+    USER ||--o{ LIKE : likes
+    USER ||--o{ ORDER : places
+
+    ORDER ||--|{ ORDER_ITEM : contains
+    PRODUCT ||--o{ ORDER_ITEM : includes
+    BRAND ||--o{ PRODUCT : owns
+    PRODUCT ||--o{ LIKE : is_liked_by
+
+    USER {
+        BIGINT id PK
+        VARCHAR(100) user_id
+        VARCHAR(255) email
+        DATE birthdate
+        INT gender
+        TIMESTAMP created_at
+        TIMESTAMP updated_at
+        TIMESTAMP deleted_at
+    }
+
+    POINT {
+        BIGINT id PK
+        BIGINT ref_user_id FK
+        BIGINT value
+        TIMESTAMP created_at
+        TIMESTAMP updated_at
+        TIMESTAMP deleted_at
+    }
+
+    LIKE {
+        BIGINT id PK
+        BIGINT ref_user_id FK
+        BIGINT ref_product_id FK
+        TIMESTAMP created_at
+        TIMESTAMP updated_at
+        TIMESTAMP deleted_at
+    }
+
+    ORDER {
+        BIGINT id PK
+        BIGINT ref_user_id FK
+        INT status
+        TIMESTAMP created_at
+        TIMESTAMP updated_at
+        TIMESTAMP deleted_at
+    }
+
+    PRODUCT {
+        BIGINT id PK
+        BIGINT ref_brand_id FK
+        VARCHAR name
+        VARCHAR description
+        BIGINT price
+        INT stock
+        TIMESTAMP created_at
+        TIMESTAMP updated_at
+        TIMESTAMP deleted_at
+    }
+
+    BRAND {
+        BIGINT id PK
+        VARCHAR name
+        VARCHAR description
+        TIMESTAMP created_at
+        TIMESTAMP updated_at
+        TIMESTAMP deleted_at
+    }
+
+    ORDER_ITEM {
+        BIGINT id PK
+        BIGINT ref_product_id FK
+        BIGINT ref_order_id FK
+        INT quantity
+        BIGINT price
+        TIMESTAMP created_at
+        TIMESTAMP updated_at
+        TIMESTAMP deleted_at
+    }
+
+```