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>

docker/environments/docker-compose.yml

@@ -1,3 +1,5 @@
+name: hrynco-notification-service
+
 services:
   migrator:
     build:
@@ -30,8 +32,24 @@ services:
     environment:
       - DOTNET_ENVIRONMENT=Production
       - App__ConnectionString=${CONNECTION_STRING}
+      - App__RabbitMq__Host=rabbitmq
+      - App__RabbitMq__User=${RABBITMQ_USER:-guest}
+      - App__RabbitMq__Password=${RABBITMQ_PASSWORD:-guest}
     depends_on:
       db:
         condition: service_started
       migrator:
-        condition: service_completed_successfully
+        condition: service_completed_successfully
+      rabbitmq:
+        condition: service_healthy
+
+  rabbitmq:
+    image: rabbitmq:4-management-alpine
+    environment:
+      RABBITMQ_DEFAULT_USER: ${RABBITMQ_USER:-guest}
+      RABBITMQ_DEFAULT_PASS: ${RABBITMQ_PASSWORD:-guest}
+    healthcheck:
+      test: ["CMD", "rabbitmq-diagnostics", "ping"]
+      interval: 10s
+      timeout: 5s
+      retries: 5