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Helpers/RequireWorkerKeyAttribute.cs

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+using System.Security.Cryptography;
+using System.Text;
+using Fitness_core_BE.Models.IOs;
+using Microsoft.AspNetCore.Mvc.Filters;
+
+namespace Fitness_core_BE.Helpers
+{
+    // Gates the subscription-billing worker endpoint behind a single shared secret in the
+    // X-Worker-Key header. Distinct from JwtMiddleware/CustomMiddleware (human/agent session
+    // auth) and from PaymentService:InternalApiKey (Core's own outbound key to Payment-Service) —
+    // this authenticates the trusted scheduler calling back into Core. Writes the 401/503
+    // response directly (like RequiresPermissionAttribute) rather than throwing, because Core
+    // has no global exception handler to turn a thrown exception into the right status code on
+    // this unauthenticated route.
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
+    public sealed class RequireWorkerKeyAttribute : Attribute, IAsyncActionFilter
+    {
+        public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next)
+        {
+            var configuration = context.HttpContext.RequestServices.GetService(typeof(IConfiguration)) as IConfiguration;
+            var configuredKey = configuration?["Worker:InternalApiKey"];
+
+            if (string.IsNullOrWhiteSpace(configuredKey))
+            {
+                await WriteErrorAsync(context, 503, "Worker:InternalApiKey is not configured.");
+                return;
+            }
+
+            var providedKey = context.HttpContext.Request.Headers["X-Worker-Key"].FirstOrDefault();
+            if (string.IsNullOrWhiteSpace(providedKey) || !FixedTimeEquals(providedKey, configuredKey))
+            {
+                await WriteErrorAsync(context, 401, "Invalid worker key.");
+                return;
+            }
+
+            await next();
+        }
+
+        private static async Task WriteErrorAsync(ActionExecutingContext context, int statusCode, string message)
+        {
+            context.HttpContext.Response.StatusCode = statusCode;
+            await context.HttpContext.Response.WriteAsJsonAsync(new ApiResponse<object>
+            {
+                StatusCode = statusCode,
+                Message = message,
+                Data = null
+            });
+        }
+
+        private static bool FixedTimeEquals(string a, string b)
+        {
+            return CryptographicOperations.FixedTimeEquals(Encoding.UTF8.GetBytes(a), Encoding.UTF8.GetBytes(b));
+        }
+    }
+}