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/Program.cs

@@ -22,6 +22,7 @@ builder.Services.AddScoped<JobService>();
 builder.Services.AddScoped<CutListPackingService>();
 builder.Services.AddScoped<ReportService>();
 builder.Services.AddScoped<PurchaseItemService>();
+builder.Services.AddScoped<CatalogService>();
 
 var app = builder.Build();