chore: strengthen backend eslint profile

src/integrations/tbank/tbank-account.policy.ts

@@ -18,8 +18,8 @@ type AccountType = keyof typeof accountTypes;
 export type SupportedTbankAccount = {
   id: string;
   name: string;
+  openedOn: null | string;
   type: (typeof accountTypes)[AccountType]['type'];
-  openedOn: string | null;
 };
 
 export function supportedTbankAccounts(
@@ -39,7 +39,7 @@ export function supportedTbankAccounts(
   });
 }
 
-function openedOn(seconds: string | number | undefined): string | null {
+function openedOn(seconds: number | string | undefined): null | string {
   if (seconds === undefined) return null;
   const date = new Date(Number(seconds) * MILLISECONDS_PER_SECOND);
   if (Number.isNaN(date.valueOf())) return null;