Hold episode highlights until the episode is in the Spotify feed; auto-fill study lesson audio from the feed by season + episode; v2.19.0

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/content.ts

@@ -71,6 +71,9 @@ export interface PodcastFeaturedLink {
   // Which series this highlight belongs to: Lane A (primary), Lane B (second), or general.
   // Missing = primary (older highlights were all about the one running series).
   lane?: 'primary' | 'second' | 'general'
+  // When true the highlight stays hidden on the public site until its season + episode number
+  // appear in the Spotify feed (the lane's season number is set on the Episode Workflow page).
+  holdUntilReleased?: boolean
   title: string
   episodeNumber?: string
   summary: string