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/Institutions/InstitutionsExtensions.cs

@@ -0,0 +1,17 @@
+namespace FiscalOS.API.Institutions;
+
+internal static class InstitutionsExtensions
+{
+  private const string RouteGroupPrefix = "/institutions";
+
+  public static RouteGroupBuilder MapInstitutionsEndpoints(this WebApplication app)
+  {
+    var institutionsGroup = app.MapGroup(RouteGroupPrefix)
+      .RequireAuthorization();
+
+    institutionsGroup.MapConnectEndpoint();
+    institutionsGroup.MapGetAvailableEndpoint();
+
+    return institutionsGroup;
+  }
+}