`MediaItem` shouldn't be an `actor`

All five stored properties are `let`. There is no isolated mutable state, so the actor buys no protection — it only forces `await` at call sites and adds hops. A `final class: Sendable` would be equivalent and simpler. (The initializers are `async` because metadata extraction is, not because of isolation.)

Locate the MediaItem declaration and inspect its five stored properties and async initializers. Update the type as described, then follow its call sites to verify unnecessary await usage is removed and the project still builds successfully.

Written by the indexing model from the issue text.