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.DAL.EF/Configurations/EmailChannelEntityConfiguration.cs

@@ -35,5 +35,9 @@ internal class EmailChannelEntityConfiguration : IEntityTypeConfiguration<EmailC
         builder.Property(x => x.IsActive).HasColumnName("is_active");
         builder.Property(x => x.Created).HasColumnName("created");
         builder.Property(x => x.Updated).HasColumnName("updated");
+
+        builder.HasMany(x => x.UsageRecords)
+            .WithOne()
+            .HasForeignKey(u => u.ProviderId);
     }
 }