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/static/css/app.css

@@ -238,6 +238,22 @@ button, input, textarea, select {
 	font-size: 2rem;
 }
 
+/* Play Indicator */
+.play-indicator {
+	position: absolute;
+	bottom: 0.5rem;
+	right: 0.5rem;
+	width: 2rem;
+	height: 2rem;
+	background: rgba(0,0,0,0.7);
+	color: #fff;
+	display: flex;
+	align-items: center;
+	justify-content: center;
+	font-size: 0.75rem;
+	pointer-events: none;
+}
+
 /* Item Tags */
 .item-tags {
 	display: flex;