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/Payment/FlatPaymentController.cs

@@ -0,0 +1,38 @@
+using Microsoft.AspNetCore.Mvc;
+
+namespace Distribution.Features.Payment;
+
+[ApiController]
+[Route("api/payments")]
+public class FlatPaymentController(PaymentService service) : ControllerBase
+{
+    [HttpGet]
+    public async Task<IActionResult> GetAll(
+        [FromQuery] Guid? storeId,
+        [FromQuery] Guid? saleId,
+        [FromQuery] int page = 1,
+        [FromQuery] int pageSize = 20)
+    {
+        var result = await service.GetAllAsync(storeId, saleId, page, pageSize);
+        return Ok(result);
+    }
+
+    [HttpGet("{id:guid}")]
+    public async Task<IActionResult> GetById(Guid id)
+    {
+        var result = await service.GetByIdAsync(id);
+        return result is null ? NotFound() : Ok(result);
+    }
+
+    [HttpPost]
+    public async Task<IActionResult> Create(CreateFlatPaymentRequest request, CancellationToken ct)
+    {
+        try
+        {
+            var inner = new CreatePaymentRequest(request.Amount, request.Method);
+            var result = await service.CreateAsync(request.SaleId, inner, ct);
+            return result is null ? NotFound(new { error = "Sale not found or access denied" }) : Ok(result);
+        }
+        catch (InvalidOperationException ex) { return BadRequest(new { error = ex.Message }); }
+    }
+}