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/Controllers/Business/EstimateApiController.cs

@@ -193,6 +193,8 @@ namespace Yavsc.Controllers
                     return BadRequest(ModelState);
                 }
             }
+            if (_context.NominativeServiceCommands.Any(command => command.InvoiceEstimateId == id))
+                return Conflict(new { Error = "An invoiced estimate cannot be deleted." });
             _context.Estimates.Remove(estimate);
             _context.SaveChanges(User.GetUserId());