fix: 修复邀请码为unique的错误

seecoder-portal-server/src/main/java/cn/seecoder/seecoderportalserver/domain/Course.java

@@ -8,10 +8,7 @@ import javax.validation.constraints.NotNull;
 import java.time.OffsetDateTime;
 
 @Entity
-@Table(
-        name = "course",
-        indexes = @Index(name = "course_code_index", columnList = "invitationCode")
-)
+@Table(name = "course")
 public class Course {
     @Id
     @GeneratedValue(strategy = GenerationType.TABLE, generator = "PK_COURSE")
@@ -53,7 +50,7 @@ public class Course {
     @CreationTimestamp
     private OffsetDateTime endTime;
 
-    @Column(unique = true)
+    @Column
     @NotNull
     private String invitationCode;