Fix-ish frontend models, a generator and a generate command

frontend-reactive/app/models/relation.ts

@@ -1,6 +1,6 @@
 import Model, { attr } from '@ember-data/model'
 
-export default class RelationModel extends Model {
+export default class Relation extends Model {
   @attr declare createdAt: Date
   @attr declare updatedAt: Date
   @attr declare fromId: string
@@ -8,3 +8,9 @@ export default class RelationModel extends Model {
   @attr declare requested: boolean
   @attr declare accepted: boolean
 }
+
+declare module 'ember-data/types/registries/model' {
+  export default interface ModelRegistry {
+    relation: Relation
+  }
+}