feat: notification system via Mercure for scraping events

- NotificationInterface: add sendInfo() and sendWarning() levels
- SymfonyNotification: implement new levels (publishes to 'notifications' topic)
- ChapterScrapingStarted: carry mangaTitle + chapterNumber, now dispatched
- ScrapeChapterHandler: dispatch ChapterScrapingStarted before scraping loop
- ScrapingEventSubscriber: wire NotificationInterface for started/scraped/failed events
- useMercureNotifications: new global Vue composable subscribing to 'notifications' topic
- App.vue: mount useMercureNotifications() at app root
- SendTestNotificationCommand: `app:notify:test --type --message` for dev/prod testing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/vue/app/App.vue

@@ -5,6 +5,9 @@
 
 <script setup>
 import NotificationToast from './shared/components/ui/NotificationToast.vue';
+import { useMercureNotifications } from './shared/composables/useMercureNotifications';
+
+useMercureNotifications();
 </script>
 
 <style>