Add support for multiple images from Twitter links

- Fetch all images from Twitter syndication API (photos array)
- Store images with unified 'image' media type (first = thumbnail by ID order)
- Display multi-image tweets in grid layout on detail page
- Add hover cycling through images on home grid (2s interval)
- Show image count indicator on multi-image items
- Extract shared downloadAndStoreImages() helper for create/refresh
- Add migration to convert existing thumbnail/gallery types to image
- Make images clickable to open in new tab on detail page

internal/migrations/sql/0004_unify_image_media_type.sql

@@ -0,0 +1,8 @@
+-- +goose Up
+-- Consolidate 'thumbnail' and 'gallery' media types into unified 'image' type
+UPDATE media SET media_type = 'image' WHERE media_type IN ('thumbnail', 'gallery');
+
+-- +goose Down
+-- Note: Cannot perfectly reverse since we lose the distinction between thumbnail and gallery
+-- This sets all 'image' back to 'thumbnail' as a fallback
+UPDATE media SET media_type = 'thumbnail' WHERE media_type = 'image';