feat: add notification events handling and MQTT integration

- Implemented notification event publishing with a new NotificationEventPublisher interface.
- Created a noopNotificationEventPublisher for testing purposes.
- Added functionality to publish notification created events via MQTT.
- Introduced a new stream event publisher for handling job logs and updates.
- Added database migration for popup_ads table.
- Created tests for notification events and popup ads functionality.
- Established MQTT connection and publishing helpers for event messages.

internal/transport/mqtt/notification_publisher.go

@@ -0,0 +1,44 @@
+package mqtt
+
+import (
+	"context"
+	"fmt"
+
+	pahomqtt "github.com/eclipse/paho.mqtt.golang"
+	"stream.api/internal/database/model"
+	"stream.api/internal/service"
+	"stream.api/pkg/logger"
+)
+
+type notificationPublisher struct {
+	client pahomqtt.Client
+	logger logger.Logger
+	prefix string
+}
+
+func NewNotificationPublisher(client pahomqtt.Client, appLogger logger.Logger) service.NotificationEventPublisher {
+	if client == nil {
+		return service.NotificationEventPublisher(serviceNotificationNoop{})
+	}
+	return &notificationPublisher{client: client, logger: appLogger, prefix: defaultMQTTPrefix}
+}
+
+type serviceNotificationNoop struct{}
+
+func (serviceNotificationNoop) PublishNotificationCreated(context.Context, *model.Notification) error { return nil }
+
+func (p *notificationPublisher) PublishNotificationCreated(_ context.Context, notification *model.Notification) error {
+	if p == nil || notification == nil {
+		return nil
+	}
+	message := mqttEvent{
+		Type:    "notification.created",
+		Payload: service.BuildNotificationCreatedPayload(notification),
+	}
+	return publishMQTTJSON(p.client, p.notificationTopic(notification.UserID), message)
+}
+
+func (p *notificationPublisher) notificationTopic(userID string) string {
+	return fmt.Sprintf("%s/notifications/%s", p.prefix, userID)
+}
+