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.Contracts/Messages/NotificationResultData.cs

@@ -0,0 +1,14 @@
+namespace HrynCo.NotificationService.Contracts.Messages;
+
+public record NotificationResultData
+{
+    public required string ServiceName { get; init; }
+    public required string RecipientEmail { get; init; }
+    public required string TemplateKey { get; init; }
+    public required DateTimeOffset Timestamp { get; init; }
+
+    /// <summary>Null when delivery succeeded; contains error details on failure.</summary>
+    public string? ErrorMessage { get; init; }
+
+    public bool IsSuccess => ErrorMessage is null;
+}