Add Twitter video support and use sql.Null[T] for nullable columns

- Store Twitter video URLs in embed_video_url column instead of downloading
- Play videos directly from Twitter's CDN in <video> element
- Fix Twitter API parsing (content_type/url fields)
- Strip t.co URLs from tweet text descriptions
- Use sql.Null[T] generic type for nullable DB columns
- Add Nullable[T] and Ptr[T] helper functions
- Add play indicator overlay for video items in grid
- Add migration for embed_video_url column

internal/migrations/sql/0003_embed_video_url.sql

@@ -0,0 +1,5 @@
+-- +goose Up
+ALTER TABLE item ADD COLUMN embed_video_url TEXT;
+
+-- +goose Down
+ALTER TABLE item DROP COLUMN embed_video_url;