feat(api,core,infra): refactor endpoint routing and add institution account discovery

- Move login and refresh endpoints under a unified /auth route group
- Relocate institution connection logic from /accounts to /institutions
- Implement GET /institutions/{id}/available to fetch real-time Plaid
accounts
- Introduce HttpRequestBuilder utility to streamline integration testing
- Enhance PlaidService with GetAccountsAsync to support account fetching
- Clean up global usings and project structure for better domain
isolation

src/FiscalOS.API/Accounts/AccountsExtensions.cs

@@ -10,7 +10,6 @@ internal static class AccountsExtensions
       .RequireAuthorization();
 
     accountsGroup.MapAddEndpoint();
-    accountsGroup.MapConnectEndpoint();
 
     return accountsGroup;
   }