Add FlatPaymentController with RESTful payment endpoints

Introduced FlatPaymentController under Distribution.Features.Payment, providing GET and POST endpoints at /api/payments. Supports listing, retrieving, and creating payments with filtering and error handling. Business logic is delegated to PaymentService.

Distribution/Features/Sale/SaleController.cs

@@ -33,4 +33,15 @@ public class SaleController(SaleService service) : ControllerBase
         var result = await service.CreateAsync(request, ct);
         return result is null ? Forbid() : CreatedAtAction(nameof(GetById), new { id = result.Id }, result);
     }
+
+    [HttpPatch("{id:guid}/cancel")]
+    public async Task<IActionResult> Cancel(Guid id, CancellationToken ct)
+    {
+        try
+        {
+            var result = await service.CancelAsync(id, ct);
+            return result is null ? NotFound() : Ok(result);
+        }
+        catch (InvalidOperationException ex) { return Conflict(new { error = ex.Message }); }
+    }
 }