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.json

@@ -1,6 +1,13 @@
 {
   "App": {
-    "ConnectionString": "Host=localhost;Port=5432;Database=notification_service;Username=postgres;Password=postgres"
+    "ConnectionString": "Host=localhost;Port=5432;Database=notification_service;Username=postgres;Password=postgres",
+    "RabbitMq": {
+      "Host": "localhost",
+      "Port": 5672,
+      "User": "guest",
+      "Password": "guest",
+      "VirtualHost": "/"
+    }
   },
   "Serilog": {
     "MinimumLevel": {