feat(persistance): add db models

src/app/models/db/DBPurchase.ts

@@ -0,0 +1,10 @@
+export class DBPurchase {
+  constructor(
+    public establishment_id: number,
+    public product_id: number,
+    public date: number,
+    public price = 0,
+    public quantity = 1,
+    public id?: number
+  ) {}
+}