feat(mind): add emotion alert entity and VO

Create EmotionAlert entity (childId, severity, title, message, source, resolved status) and EmotionAlertVO for API responses.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

cfc-backend/src/main/java/com/etotem/cfc/dto/EmotionAlertVO.java

@@ -0,0 +1,18 @@
+package com.etotem.cfc.dto;
+
+import lombok.Data;
+
+import java.io.Serializable;
+import java.util.Date;
+
+@Data
+public class EmotionAlertVO implements Serializable {
+    private Long id;
+    private Long childId;
+    private String alertType;
+    private String severity;
+    private String title;
+    private String message;
+    private Boolean resolved;
+    private Date createdAt;
+}

cfc-backend/src/main/java/com/etotem/cfc/entity/EmotionAlert.java

@@ -0,0 +1,35 @@
+package com.etotem.cfc.entity;
+
+import com.baomidou.mybatisplus.annotation.IdType;
+import com.baomidou.mybatisplus.annotation.TableId;
+import com.baomidou.mybatisplus.annotation.TableName;
+import lombok.Data;
+
+import java.io.Serializable;
+import java.util.Date;
+
+@Data
+@TableName("emotion_alert")
+public class EmotionAlert implements Serializable {
+
+    @TableId(type = IdType.AUTO)
+    private Long id;
+
+    private Long childId;
+
+    private String alertType;          // consecutive_low_mood / crisis_keyword / sharp_decline
+
+    private String severity;           // low / medium / high
+
+    private String title;              // 告警标题
+
+    private String message;            // 告警具体描述
+
+    private Long relatedCheckinId;     // 关联的情绪打卡ID（可为null）
+
+    private Boolean resolved;          // 是否已处理
+
+    private Date createdAt;
+
+    private Date resolvedAt;
+}