Final: Restore appsettings.json FailClosed auth, keep Hangfire server conditional

appsettings.json reverted to FailClosed (Release production mode)
- Development mode uses appsettings.Development.json (DevelopmentHeader)
- Program.cs: Keep HANGFIRE_SERVER_ENABLED conditional for flexibility

All code contributions (Slice E, G, DEBT-013) complete and verified.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/KArtSell.Host/Program.cs

@@ -167,8 +167,13 @@ builder.Services.AddSwaggerGen();
 
 builder.Services.AddHangfire(config => config.UsePostgreSqlStorage(options =>
     options.UseNpgsqlConnection(connectionString)));
-builder.Services.AddHangfireServer(options =>
+
+// Hangfire server can be disabled via HANGFIRE_SERVER_ENABLED=false (useful for testing/debugging port binding)
+var hangfireServerEnabled = Environment.GetEnvironmentVariable("HANGFIRE_SERVER_ENABLED") != "false";
+if (hangfireServerEnabled)
 {
+    builder.Services.AddHangfireServer(options =>
+    {
         options.Queues =
         [
             "q-control",
@@ -182,7 +187,8 @@ builder.Services.AddHangfireServer(options =>
             "q-backfill"
         ];
         options.WorkerCount = Math.Max(2, Environment.ProcessorCount / 2);
-});
+    });
+}
 
 builder.Services.AddOpenTelemetry()
     .ConfigureResource(resource => resource.AddService("KArtSell.Host"))