feat: limit daily AI formatting usage

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

@@ -0,0 +1,31 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+import java.time.LocalDate;
+
+/** Daily count of AI markdown formatting usage for a user. */
+@Entity
+@Getter
+@Setter
+@NoArgsConstructor
+@Table(name = "ai_format_usage",
+       uniqueConstraints = @UniqueConstraint(columnNames = {"user_id", "use_date"}))
+public class AiFormatUsage {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "user_id")
+    private User user;
+
+    @Column(name = "use_date", nullable = false)
+    private LocalDate useDate;
+
+    @Column(nullable = false)
+    private int count;
+}