chore: add missing javadoc

Author: Sporiff

src/main/java/org/openpodcastapi/opa/pagination/Cursorable.java

@@ -5,6 +5,13 @@
 /// An interface for results that can be paginated using a pagination.
 /// Any cursorable entity must have methods for fetching the database ID and created timestamp.
 public interface Cursorable {
+    /**
+     * @return the entity ID
+     */
     Long getId();
+
+    /**
+     * @return the entity's `created_at` timestamp
+     */
     Instant getCreatedAt();
 }