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/handlers/api_items.go

@@ -118,13 +118,13 @@ func HandleCreateItem(rc *RequestContext, w http.ResponseWriter, r *http.Request
 	defer cancel()
 
 	it, err := item.QCreate(ctx, rc.DB, item.CreateParams{
-		Title:         req.Title,
-		Description:   req.Description,
-		LinkURL:       req.LinkURL,
+		Title:         item.Nullable(req.Title),
+		Description:   item.Nullable(req.Description),
+		LinkURL:       item.Nullable(req.LinkURL),
 		ItemType:      req.ItemType,
-		EmbedProvider: req.EmbedProvider,
-		EmbedVideoID:  req.EmbedVideoID,
-		EmbedHTML:     req.EmbedHTML,
+		EmbedProvider: item.Nullable(req.EmbedProvider),
+		EmbedVideoID:  item.Nullable(req.EmbedVideoID),
+		EmbedHTML:     item.Nullable(req.EmbedHTML),
 	})
 	if err != nil {
 		return err
@@ -173,9 +173,9 @@ func HandleUpdateItem(rc *RequestContext, w http.ResponseWriter, r *http.Request
 	}
 
 	if err := item.QUpdate(ctx, rc.DB, it.ID, item.UpdateParams{
-		Title:       req.Title,
-		Description: req.Description,
-		LinkURL:     req.LinkURL,
+		Title:       item.Nullable(req.Title),
+		Description: item.Nullable(req.Description),
+		LinkURL:     item.Nullable(req.LinkURL),
 	}); err != nil {
 		return err
 	}
@@ -241,11 +241,11 @@ func buildItemResponse(ctx context.Context, rc *RequestContext, it item.Row) (it
 
 	resp := itemResponse{
 		ID:          it.PubID,
-		Title:       it.Title,
-		Description: it.Description,
-		LinkURL:     it.LinkURL,
+		Title:       item.Ptr(it.Title),
+		Description: item.Ptr(it.Description),
+		LinkURL:     item.Ptr(it.LinkURL),
 		ItemType:    it.ItemType,
-		EmbedHTML:   it.EmbedHTML,
+		EmbedHTML:   item.Ptr(it.EmbedHTML),
 		Tags:        tagNames,
 		CreatedAt:   it.CreatedAt.Format(time.RFC3339),
 	}