perf(backend+frontend): 列表API响应体积优化 3.1MB→145KB (↓95.4%)

- 后端: JPQL构造器投影排除LONGTEXT大字段(uploadedImages/videoReferenceImages)
- 后端: DTO层过滤非分镜图类型的base64内联resultUrl
- 前端: 列表缩略图从video改为img loading=lazy，消除172并发请求
- 前端: download函数增加resultUrl懒加载(详情接口兜底)
- 文档: 新增性能优化报告 docs/performance-optimization-report.md

src/main/java/com/example/demo/dto/MailMessage.java

@@ -0,0 +1,82 @@
+package com.example.demo.dto;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * 邮件消息DTO
+ * 用于封装邮件发送请求
+ */
+public class MailMessage {
+    
+    private String toEmail;
+    private String subject;
+    private Long templateId;
+    private Map<String, Object> templateData;
+    
+    public MailMessage() {
+        this.templateData = new HashMap<>();
+    }
+    
+    public MailMessage(String toEmail, String subject, Long templateId) {
+        this();
+        this.toEmail = toEmail;
+        this.subject = subject;
+        this.templateId = templateId;
+    }
+    
+    public String getToEmail() {
+        return toEmail;
+    }
+    
+    public void setToEmail(String toEmail) {
+        this.toEmail = toEmail;
+    }
+    
+    public String getSubject() {
+        return subject;
+    }
+    
+    public void setSubject(String subject) {
+        this.subject = subject;
+    }
+    
+    public Long getTemplateId() {
+        return templateId;
+    }
+    
+    public void setTemplateId(Long templateId) {
+        this.templateId = templateId;
+    }
+    
+    public Map<String, Object> getTemplateData() {
+        return templateData;
+    }
+    
+    public void setTemplateData(Map<String, Object> templateData) {
+        this.templateData = templateData;
+    }
+    
+    /**
+     * 添加模板参数
+     */
+    public MailMessage addParam(String key, Object value) {
+        this.templateData.put(key, value);
+        return this;
+    }
+}
+
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