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/AppSettings.cs

@@ -1,3 +1,5 @@
+using Hrynco.RabbitMq;
+
 namespace HrynCo.NotificationService.Worker;
 
 public sealed class AppSettings
@@ -5,4 +7,5 @@ public sealed class AppSettings
     public const string SectionName = "App";
 
     public string ConnectionString { get; init; } = string.Empty;
+    public RabbitMqSettings RabbitMq { get; init; } = null!;
 }