Add activity module with milk tea event

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

@@ -0,0 +1,48 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+import org.hibernate.annotations.CreationTimestamp;
+
+import java.time.LocalDateTime;
+import java.util.HashSet;
+import java.util.Set;
+
+/** Generic activity entity. */
+@Entity
+@Getter
+@Setter
+@NoArgsConstructor
+@Table(name = "activities")
+public class Activity {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @Column(nullable = false)
+    private String title;
+
+    private String icon;
+
+    @Column(name = "start_time", nullable = false)
+    @CreationTimestamp
+    private LocalDateTime startTime;
+
+    @Column(name = "end_time")
+    private LocalDateTime endTime;
+
+    @Enumerated(EnumType.STRING)
+    @Column(nullable = false)
+    private ActivityType type = ActivityType.NORMAL;
+
+    @ManyToMany(fetch = FetchType.LAZY)
+    @JoinTable(name = "activity_participants",
+            joinColumns = @JoinColumn(name = "activity_id"),
+            inverseJoinColumns = @JoinColumn(name = "user_id"))
+    private Set<User> participants = new HashSet<>();
+
+    @Column(nullable = false)
+    private boolean ended = false;
+}