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/service/notification_events.go

@@ -0,0 +1,57 @@
+package service
+
+import (
+	"context"
+	"encoding/json"
+	"time"
+
+	"stream.api/internal/database/model"
+)
+
+type noopNotificationEventPublisher struct{}
+
+func (noopNotificationEventPublisher) PublishNotificationCreated(context.Context, *model.Notification) error {
+	return nil
+}
+
+type NotificationCreatedPayload struct {
+	ID          string  `json:"id"`
+	UserID      string  `json:"user_id"`
+	Type        string  `json:"type"`
+	Title       string  `json:"title"`
+	Message     string  `json:"message"`
+	ActionURL   *string `json:"action_url,omitempty"`
+	ActionLabel *string `json:"action_label,omitempty"`
+	CreatedAt   string  `json:"created_at,omitempty"`
+}
+
+func (s *appServices) publishNotificationCreated(ctx context.Context, notification *model.Notification) {
+	if s == nil || s.notificationEvents == nil || notification == nil {
+		return
+	}
+	if err := s.notificationEvents.PublishNotificationCreated(ctx, notification); err != nil {
+		s.logger.Error("Failed to publish notification MQTT event", "error", err, "notification_id", notification.ID, "user_id", notification.UserID)
+	}
+}
+
+func BuildNotificationCreatedPayload(notification *model.Notification) NotificationCreatedPayload {
+	createdAt := ""
+	if notification != nil && notification.CreatedAt != nil {
+		createdAt = notification.CreatedAt.UTC().Format(time.RFC3339)
+	}
+	return NotificationCreatedPayload{
+		ID:          notification.ID,
+		UserID:      notification.UserID,
+		Type:        notification.Type,
+		Title:       notification.Title,
+		Message:     notification.Message,
+		ActionURL:   nullableTrimmedString(notification.ActionURL),
+		ActionLabel: nullableTrimmedString(notification.ActionLabel),
+		CreatedAt:   createdAt,
+	}
+}
+
+func mustMarshalNotificationPayload(notification *model.Notification) []byte {
+	encoded, _ := json.Marshal(BuildNotificationCreatedPayload(notification))
+	return encoded
+}