[editor] Asset tagging - tags on images, text, and video

- Database: create `tags` + `asset_tags` join tables (normalized approach)

- Add TagInput to image, text, and video edit panels

- Asset gallery: tag display on thumbnails, filtering by tag, bulk tag assignment

- Tag cloud of used tags

- Sort by tag, date, size, name

**Files:** Supabase migration, `ImageEditPanel.tsx`, `TextEditPanel.tsx`, `VideoEditPanel.tsx`, `AssetGallery.tsx`