Fix banner service by populating Order.confirmedAt timestamp

[Copilot]

Problem

The banner service queries orders by confirmedAt to determine review prompt eligibility, but this field was never populated. All banner queries returned empty results.

Changes

Added confirm() method to Order entity:

public void confirm() {
    this.orderStatus = OrderStatus.CONFIRMED;
    this.confirmedAt = LocalDateTime.now();
}

Follows existing cancel() pattern. Enables banner service to:

• Show initial review prompts after order confirmation
• Show monthly review prompts 5+ days post-confirmation

Modified: src/main/java/com/ongil/backend/domain/order/entity/Order.java

Original prompt

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This section details on the original issue you should resolve

<issue_title>[fix] 홈화면 배너</issue_title>
<issue_description>## 📌 Description

[fix] 홈화면 배너

✅ To-do

• 진행 예정
• 진행 완료

📒 기타</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [fix] 홈화면 배너 #129

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