feat(persistance): add db models

src/app/models/db/DBChain.ts

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+import { Chain } from '../Chain';
+
+export class DBChain extends Chain {}

src/app/models/db/DBEstablishment.ts

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+export class DBEstablishment {
+  constructor(public address: string, public chain_id: number, public id?: number) {}
+}

src/app/models/db/DBProduct.ts

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+import { Product } from '../Product';
+
+export class DBProduct extends Product {}

src/app/models/db/DBProductEstablishment.ts

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+export class DBProductEstablishment {
+  constructor(public product_id: number, public establishment_id: number, public id?: number) {}
+}

src/app/models/db/DBPurchase.ts

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