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/Activities/Pictures/UpdatePlayListPictureActivity.cs

@@ -11,7 +11,7 @@ namespace Musify.Infrastructure.MassTransit.Activities.Pictures
         ILogger<UpdatePlayListPictureActivity> logger)
         : IActivity<UpdatePlayListPictureArguments, UpdatePlayListPictureLog>
     {
-        public const string ExecuteEndpointName = "update-playlist-picture";
+        public const string ExecuteEndpointName = "update-play-list-picture";
 
         public async Task<ExecutionResult> Execute(ExecuteContext<UpdatePlayListPictureArguments> executeContext)
         {

backend/Musify.Infrastructure/MassTransit/Activities/PlayLists/DeletePlayListFromDbActivity.cs

@@ -11,7 +11,7 @@ namespace Musify.Infrastructure.MassTransit.Activities.PlayLists
         ILogger<DeletePlayListFromDbActivity> logger)
         : IExecuteActivity<DeletePlayListFromDbArguments>
     {
-        public const string ExecuteEndpointName = "delete-playList-from-db";
+        public const string ExecuteEndpointName = "delete-play-list-from-db";
 
         public async Task<ExecutionResult> Execute(ExecuteContext<DeletePlayListFromDbArguments> executeContext)
         {

backend/Musify.Infrastructure/MassTransit/Activities/PlayLists/MarkPlayListAsRemovingActivity.cs

@@ -12,7 +12,7 @@ namespace Musify.Infrastructure.MassTransit.Activities.PlayLists
         ILogger<MarkPlayListAsRemovingActivity> logger)
         : IExecuteActivity<MarkPlayListAsRemovingArguments>
     {
-        public const string ExecuteEndpointName = "mark-playList-as-removing";
+        public const string ExecuteEndpointName = "mark-play-list-as-removing";
 
         public async Task<ExecutionResult> Execute(ExecuteContext<MarkPlayListAsRemovingArguments> executeContext)
         {

backend/Musify.Infrastructure/MassTransit/Activities/PlayLists/PublishPlayListPictureProcessingEventActivity.cs

@@ -10,7 +10,7 @@ namespace Musify.Infrastructure.MassTransit.Activities.PlayLists
         ILogger<PublishPlayListPictureProcessingEventActivity> logger)
         : IExecuteActivity<PublishPlayListPictureProcessingEventArguments>
     {
-        public const string ExecuteEndpointName = "publish-playList-picture-processing-event";
+        public const string ExecuteEndpointName = "publish-play-list-picture-processing-event";
 
         public async Task<ExecutionResult> Execute(ExecuteContext<PublishPlayListPictureProcessingEventArguments> executeContext)
         {