Implement existing API types

frontend-reactive/app/models/relation.ts

@@ -0,0 +1,10 @@
+import Model, { attr } from '@ember-data/model'
+
+export default class RelationModel extends Model {
+  @attr declare createdAt: Date
+  @attr declare updatedAt: Date
+  @attr declare fromId: string
+  @attr declare toId: string
+  @attr declare requested: boolean
+  @attr declare accepted: boolean
+}