feat(invoicing): connect signed estimates and verified payment receipts

Freeze invoice totals and parties, record offline receipts and verify completed PayPal transactions. Protect idempotence and concurrent balance updates; add migration, API client, documentation and tests.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/Yavsc.Api/Program.cs

@@ -72,6 +72,7 @@ internal class Program
         // the document-generation flow and BillingController.
         services.Configure<SiteSettings>(builder.Configuration.GetSection("Site"));
         services.AddPodcasts(builder.Configuration);
+        services.AddInvoicing(builder.Configuration);
 
         // accepts any access token issued by identity server
         services.AddAuthentication("Bearer")