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/Auth/Login/Response.cs

@@ -0,0 +1,22 @@
+namespace FiscalOS.API.Auth.Login;
+
+internal sealed record Response
+{
+  public string AccessToken { get; init; }
+
+  [JsonConstructor]
+  private Response()
+  {
+    AccessToken = string.Empty;
+  }
+
+  private Response(string accessToken)
+  {
+    AccessToken = accessToken;
+  }
+
+  public static Response From(string accessToken)
+  {
+    return new(accessToken);
+  }
+}