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/model/UserActivityStats.java

@@ -0,0 +1,84 @@
+package com.example.demo.model;
+
+import jakarta.persistence.*;
+import java.math.BigDecimal;
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+
+@Entity
+@Table(name = "user_activity_stats")
+public class UserActivityStats {
+    
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+    
+    @Column(name = "activity_date", nullable = false, unique = true)
+    private LocalDate activityDate;
+    
+    @Column(name = "daily_active_users", nullable = false)
+    private Integer dailyActiveUsers = 0;
+    
+    @Column(name = "monthly_active_users", nullable = false)
+    private Integer monthlyActiveUsers = 0;
+    
+    @Column(name = "new_users", nullable = false)
+    private Integer newUsers = 0;
+    
+    @Column(name = "returning_users", nullable = false)
+    private Integer returningUsers = 0;
+    
+    @Column(name = "session_count", nullable = false)
+    private Integer sessionCount = 0;
+    
+    @Column(name = "avg_session_duration", precision = 10, scale = 2)
+    private BigDecimal avgSessionDuration = BigDecimal.ZERO;
+    
+    @Column(name = "created_at", nullable = false)
+    private LocalDateTime createdAt;
+    
+    @Column(name = "updated_at")
+    private LocalDateTime updatedAt;
+    
+    @PrePersist
+    protected void onCreate() {
+        createdAt = LocalDateTime.now();
+        updatedAt = LocalDateTime.now();
+    }
+    
+    @PreUpdate
+    protected void onUpdate() {
+        updatedAt = LocalDateTime.now();
+    }
+    
+    // Getters and Setters
+    public Long getId() { return id; }
+    public void setId(Long id) { this.id = id; }
+    
+    public LocalDate getActivityDate() { return activityDate; }
+    public void setActivityDate(LocalDate activityDate) { this.activityDate = activityDate; }
+    
+    public Integer getDailyActiveUsers() { return dailyActiveUsers; }
+    public void setDailyActiveUsers(Integer dailyActiveUsers) { this.dailyActiveUsers = dailyActiveUsers; }
+    
+    public Integer getMonthlyActiveUsers() { return monthlyActiveUsers; }
+    public void setMonthlyActiveUsers(Integer monthlyActiveUsers) { this.monthlyActiveUsers = monthlyActiveUsers; }
+    
+    public Integer getNewUsers() { return newUsers; }
+    public void setNewUsers(Integer newUsers) { this.newUsers = newUsers; }
+    
+    public Integer getReturningUsers() { return returningUsers; }
+    public void setReturningUsers(Integer returningUsers) { this.returningUsers = returningUsers; }
+    
+    public Integer getSessionCount() { return sessionCount; }
+    public void setSessionCount(Integer sessionCount) { this.sessionCount = sessionCount; }
+    
+    public BigDecimal getAvgSessionDuration() { return avgSessionDuration; }
+    public void setAvgSessionDuration(BigDecimal avgSessionDuration) { this.avgSessionDuration = avgSessionDuration; }
+    
+    public LocalDateTime getCreatedAt() { return createdAt; }
+    public void setCreatedAt(LocalDateTime createdAt) { this.createdAt = createdAt; }
+    
+    public LocalDateTime getUpdatedAt() { return updatedAt; }
+    public void setUpdatedAt(LocalDateTime updatedAt) { this.updatedAt = updatedAt; }
+}