Add image reference tracking

src/main/java/com/openisle/model/Image.java

@@ -0,0 +1,26 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+/**
+ * Image entity tracking COS image reference counts.
+ */
+@Entity
+@Getter
+@Setter
+@NoArgsConstructor
+@Table(name = "images")
+public class Image {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @Column(nullable = false, unique = true, length = 512)
+    private String url;
+
+    @Column(nullable = false)
+    private long refCount = 0;
+}