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.Infra/Accounts/Plaid/PlaidService.cs

@@ -1,3 +1,4 @@
+
 namespace FiscalOS.Infra.Accounts.Plaid;
 
 public sealed class PlaidService
@@ -30,6 +31,21 @@ public sealed class PlaidService
     return (ptr.ItemId, ptr.AccessToken);
   }
 
+  public async Task<List<Going.Plaid.Entity.Account>> GetAccountsAsync(string accessToken)
+  {
+    var ar = await _client.AccountsGetAsync(new()
+    {
+      AccessToken = accessToken,
+    }).ConfigureAwait(false);
+
+    if (ar.IsSuccessStatusCode is false)
+    {
+      throw new PlaidException("Unable to retrieve accounts");
+    }
+
+    return [.. ar.Accounts];
+  }
+
   public async Task<ItemWithConsentFields> GetItemAsync(string accessToken)
   {
     var ar = await _client.ItemGetAsync(new()
@@ -44,4 +60,4 @@ public sealed class PlaidService
 
     return ar.Item;
   }
-}
+}