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.Services/EmailChannels/Send/SendEmailCommand.cs

@@ -0,0 +1,17 @@
+using HrynCo.NotificationService.Services.Core;
+using MediatR;
+
+namespace HrynCo.NotificationService.Services.EmailChannels.Send;
+
+/// <summary>
+/// Sends an email via the channel associated with the given channel ID,
+/// then increments the usage counter for that channel.
+/// </summary>
+public sealed record SendEmailCommand(
+    Guid ChannelId,
+    string RecipientEmail,
+    string RecipientName,
+    string Subject,
+    string HtmlBody,
+    string? TextBody
+) : IRequest<ServiceResult<Core.Unit>>;