Remove ShopWorker entity and migrate worker identity to ApplicationUser

Removes the ShopWorker and ShopWorkerRoleCost entities, all related DTOs,
mappings, controllers, views, and import/export paths. Worker identity is
now handled entirely through ApplicationUser with per-user LaborCostPerHour.
ShopWorkerRoleCosts table remains in production pending manual data migration.

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

src/PowderCoating.Web/Controllers/DataPurgeController.cs

@@ -352,7 +352,7 @@ public class DataPurgeController : Controller
             "MaintenanceRecords",
             "Jobs", "Customers", "Quotes",
             "InventoryItems", "Equipment",
-            "Vendors", "ShopWorkers"
+            "Vendors"
         };
         return order.Where(entities.Contains).ToArray();
     }