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

tests/FiscalOS.API.Tests/Usings.cs

@@ -27,6 +27,7 @@ global using Microsoft.AspNetCore.Mvc;
 global using Microsoft.AspNetCore.Mvc.Testing;
 global using Microsoft.AspNetCore.TestHost;
 global using Microsoft.EntityFrameworkCore;
+global using Microsoft.Extensions.Configuration;
 global using Microsoft.Extensions.DependencyInjection;
 global using Microsoft.Extensions.Logging;
 global using Microsoft.Extensions.Options;