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/Add/Endpoint.cs

@@ -1,5 +1,3 @@
-using FiscalOS.Core.Accounts;
-
 namespace FiscalOS.API.Accounts.Add;
 
 internal static class Endpoint

src/FiscalOS.API/Accounts/Add/Request.cs

@@ -1,4 +1,3 @@
-
 namespace FiscalOS.API.Accounts.Add;
 
 public record Request : IValidatableObject