feat: add RabbitMQ worker, contracts, usage UI in channels screen

- Add HrynCo.NotificationService.Contracts project with SendEmailMessage and NotificationResultMessage
- Add SendEmailConsumer (RabbitMQ worker) with reply-to pattern via CorrelationContext.ReplyTo
- Add SendEmailHandler owning SMTP send + usage increment as business logic
- Add GetChannelUsageSummaryHandler with single DB query via navigation property
- Merge usage stats inline into channels list (daily/monthly with progress bars)
- Refactor AdminChannelsController.Index to use GetChannelUsageSummaryQuery
- Add RabbitMQ service to docker-compose files
- Remove dead AdminChannelUsageController, ChannelUsageViewModel, ChannelUsageSummary

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.Worker/Program.cs

@@ -1,6 +1,8 @@
 using HrynCo.NotificationService.DAL.EF;
 using HrynCo.NotificationService.Services;
 using HrynCo.NotificationService.Worker;
+using Hrynco.RabbitMq;
+using Microsoft.Extensions.Options;
 
 var builder = Host.CreateApplicationBuilder(args);
 
@@ -13,7 +15,11 @@ var appSettings = builder.Configuration
 builder.Services.AddSingleton(appSettings);
 builder.Services.AddNotificationDataAccess(appSettings.ConnectionString);
 builder.Services.AddNotificationServices();
-builder.Services.AddHostedService<Worker>();
+
+builder.Services.Configure<RabbitMqSettings>(
+    builder.Configuration.GetSection($"{AppSettings.SectionName}:RabbitMq"));
+
+builder.Services.AddHostedService<SendEmailConsumer>();
 
 var host = builder.Build();
 host.Run();