added Creating Groups to frontend and saving them in Database. Also prepared some Methods for Snapshot db

src/main/java/mops/gruppen2/domain/EventDTO.java

@@ -1,7 +1,6 @@
 package mops.gruppen2.domain;
 
 import lombok.Data;
-import lombok.Value;
 import org.springframework.data.annotation.Id;
 import org.springframework.data.relational.core.mapping.Table;
 
@@ -9,8 +8,8 @@ import org.springframework.data.relational.core.mapping.Table;
 @Data
 public class EventDTO {
     @Id
-    long event_id;
-    long group_id;
+    Long event_id;
+    Long group_id;
     String user_id;
     String event_payload;
 }