perf: reduce dashboard loading overhead

[tabuna]

Summary

• count unread notifications in SQL instead of hydrating the relation
• cap the initial badge query at 10 because the frontend replaces two-digit counts with an indicator
• fetch only notification IDs and remove unnecessary notification ordering
• preserve custom notification relations by avoiding a hard-coded table name
• cover the numeric-to-indicator transition with a Dusk test
• create notification fixtures declaratively through a dedicated test factory
• lazily load example images

Tests

• php vendor/bin/pint --test src/Platform/Components/Notification.php src/Platform/Http/Controllers/NotificationController.php tests/Feature/Platform/NotificationTest.php tests/Browser/NotificationTest.php tests/Database/Factory/DatabaseNotificationFactory.php
• php vendor/bin/phpunit tests/Feature/Platform/NotificationTest.php
• php vendor/bin/phpunit --testsuite=Browser --filter=NotificationTest --no-coverage