feat: Add catalog import/export API endpoints

Replace the standalone ExportData console app and hardcoded SeedController
with generic GET /api/catalog/export and POST /api/catalog/import endpoints.
Import uses upsert semantics with per-item error handling, preserving
existing inventory quantities.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CutList.Web/Controllers/CatalogController.cs

@@ -0,0 +1,41 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using CutList.Web.DTOs;
+using CutList.Web.Services;
+using Microsoft.AspNetCore.Mvc;
+
+namespace CutList.Web.Controllers;
+
+[ApiController]
+[Route("api/[controller]")]
+public class CatalogController : ControllerBase
+{
+    private readonly CatalogService _catalogService;
+
+    public CatalogController(CatalogService catalogService)
+    {
+        _catalogService = catalogService;
+    }
+
+    [HttpGet("export")]
+    public async Task<IActionResult> Export()
+    {
+        var data = await _catalogService.ExportAsync();
+
+        var options = new JsonSerializerOptions
+        {
+            WriteIndented = true,
+            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+            PropertyNamingPolicy = JsonNamingPolicy.CamelCase
+        };
+
+        return new JsonResult(data, options);
+    }
+
+    [HttpPost("import")]
+    public async Task<ActionResult<ImportResultDto>> Import([FromBody] CatalogData data)
+    {
+        var result = await _catalogService.ImportAsync(data);
+        return Ok(result);
+    }
+}