Add alt text to Small Y demo media images

The Small Y demo content creates 133 image media (small_y_demo_media_image) without alternative text, so every new site installed with the Small Y preset starts with empty alt attributes. This adds an alt key to 132 of 133 rows (svg_404_image is left without alt) and maps it to field_media_image/alt; field_media_image becomes field_media_image/target_id with the same migration_lookup, as y_lb_demo_content does for icon media. Testing: fresh install with the Small Y preset, then SELECT COUNT(*) FROM media__field_media_image WHERE field_media_image_alt <> ''; returns 132 (was 0), migrate_message_small_y_demo_media_image is empty.