Added unique constraint on subscriber's email.

Signed-off-by: Pavel Kirilin <win10@list.ru>

src/main/kotlin/com/s3ai/corporate_app2/jms/persistance/entities.kt

@@ -48,7 +48,7 @@ class Subscriber {
     @GeneratedValue(strategy = GenerationType.AUTO)
     var id: UUID? = null
 
-    @Column(name = "email")
+    @Column(name = "email", unique = true)
     var email: String? = null
 
     @CreationTimestamp