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/HelpController.cs

@@ -38,14 +38,6 @@ namespace PowderCoating.Web.Controllers
             return View();
         }
 
-        /// <summary>
-        /// Serves the Shop Workers help article describing roles, assignment to jobs, and maintenance tasks.
-        /// </summary>
-        public IActionResult ShopWorkers()
-        {
-            return View();
-        }
-
         /// <summary>
         /// Serves the Equipment help article explaining the equipment status lifecycle and maintenance scheduling.
         /// </summary>