refactor(infra): auto-configure MassTransit endpoints via ConfigureEndpoints

Replace ~240 lines of hand-written per-endpoint wiring with MassTransit's
ConfigureEndpoints + KebabCaseEndpointNameFormatter, and apply retry + EF
outbox globally through AddConfigureEndpointsCallback. Adding a consumer or
activity is now a single AddConsumer/AddActivity line.

- Normalize the 4 mismatched activity endpoint names to kebab-case so the
  routing-slip addressing keeps resolving (update-play-list-picture,
  delete/mark/publish play-list variants); cleanup consumer queue is now
  routing-slip-clean-up
- Drop the now-unused QueueName consts from event consumers and trim
  EndpointHelper / delete the manual endpoint-config partial

Verified end-to-end: booted the Worker against RabbitMQ in Docker and
confirmed all 38 endpoints are configured with the exact names the builders
address (19 execute queues + compensations + consumers + routing-slip-clean-up).

Build: 0 warnings, 0 errors.

backend/Musify.Infrastructure/MassTransit/DependencyInjection/MassTransitDependencyInjection.cs

@@ -62,15 +62,37 @@ namespace Musify.Infrastructure.MassTransit
                     outbox.DuplicateDetectionWindow = TimeSpan.FromMinutes(30);
                 });
 
+                options.SetKebabCaseEndpointNameFormatter();
+
                 RegisterConsumersAndActivities(options);
 
+                options.AddConfigureEndpointsCallback((registrationContext, _, endpointConfigurator) =>
+                {
+                    UseStandardRetry(endpointConfigurator);
+                    endpointConfigurator.UseEntityFrameworkOutbox<Database>(registrationContext);
+                });
+
                 options.UsingRabbitMq((busRegistrationContext, busFactoryConfigurator) =>
-                    ConfigureRabbitMqWorkerEndpoints(busRegistrationContext, busFactoryConfigurator));
+                {
+                    ConfigureRabbitMqHost(
+                        busFactoryConfigurator,
+                        busRegistrationContext.GetRequiredService<MassTransitConfiguration>());
+
+                    busFactoryConfigurator.ConfigureEndpoints(busRegistrationContext);
+                });
             } );
 
             return serviceDescriptors;
         }
 
+        static void UseStandardRetry(IReceiveEndpointConfigurator endpointConfigurator) =>
+            endpointConfigurator.UseMessageRetry(retryConfigurator =>
+                retryConfigurator.Exponential(
+                    retryLimit: 5,
+                    minInterval: TimeSpan.FromSeconds(1),
+                    maxInterval: TimeSpan.FromSeconds(30),
+                    intervalDelta: TimeSpan.FromSeconds(2)));
+
         static void ConfigureRabbitMqHost(
             IRabbitMqBusFactoryConfigurator busFactoryConfigurator,
             MassTransitConfiguration massTransitConfiguration)