Replace duck-type check in `likes-row.tsx` with a discriminated union on `TmdbItem` and `iTunesItem`

[ooloth]

Why

likes-row.tsx discriminates between TmdbItem and iTunesItem at runtime using 'artist' in item, a structural duck-type check that is invisible to the type system — a future field change could silently break the discrimination without a compile error.

Current state

ui/sections/likes-row.tsx line 34 uses const isItunesItem = 'artist' in item. Both TmdbItem and iTunesItem share fields id, title, date, imageUrl, link with no type discriminant field, so the runtime check is the only way to distinguish them.

Ideal state

• TmdbItem carries type: 'tmdb' and iTunesItem carries type: 'itunes' as discriminant fields.
• The prop is typed as a proper tagged union TmdbItem | iTunesItem.
• The isItunesItem check is replaced by a switch (item.type) or equivalent that TypeScript narrows correctly.
• A future field change does not silently break the discrimination.

Starting points

• ui/sections/likes-row.tsx — line 34 (the duck-type check to replace)
• Type definitions for TmdbItem and iTunesItem — where the discriminant fields should be added

QA plan

1. Open ui/sections/likes-row.tsx and verify no 'artist' in item check exists.
2. Remove the type discriminant field from iTunesItem — expect a compile error in the component.
3. Run tsc --noEmit — expect no type errors with both discriminant fields present.

Done when

TmdbItem | iTunesItem is a discriminated union with a type field and the discrimination in likes-row.tsx is type-narrowing rather than a structural check.