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/Program.cs

@@ -32,11 +32,8 @@ app.UseAuthentication();
 app.UseAuthorization();
 app.UseStatusCodePages();
 
-app.MapLoginEndpoint();
-
-app.MapRefreshEndpoint()
-  .RequireAuthorization(Schemes.AllowExpiredTokens);
-
+app.MapAuthEndpoints();
 app.MapAccountsEndpoints();
+app.MapInstitutionsEndpoints();
 
 app.Run();