YouTube & content models

File	Stores
`video.model.ts`	Canonical YouTube video row
`video-timestamp.model.ts`	Admin/curated timestamps on a video
`crowd-video-timestamp.model.ts`	User-submitted timestamp candidates
`question.model.ts`	MCQ tied to video content
`playlist.model.ts`	Curated playlists
`content.model.ts`	Announcements / exam info style content
`mock-test.model.ts`	Mock test definitions
`user-library-channel.model.ts`	Student saved channel
`user-library-video.model.ts`	Student saved video
`user-playlist.model.ts`	Student playlist
`marketing-deal.model.ts`	Sponsor deal
`marketing-sponsor.model.ts`	Sponsor entity