Move persistence I/O off the main actor

`JSONDocumentStore`'s methods are synchronous, and every caller is `@MainActor`:

- `saveNowPlayingSnapshotNow()` — on pause, on backgrounding, and from the debounced task
- `saveHistoryNow()` — **on every recorded play**
- `loadIfNeeded()` / `loadAllSavedPlaylists()` — at launch

Each call is `JSONEncoder.encode` + atomic `Data.write`, or `Data(contentsOf:)` + decode. Because `MediaReference` carries multi-kilobyte bookmark `Data` that base64-inflates in JSON, a queue of a few hundred tracks is a multi-megabyte encode-and-write **on the main thread**.

History is the sharpest edge: `saveHistoryNow()` rewrites the *entire* history file every time a track crosses the one-second threshold. With `.forever` retention (the default) that file only grows.

`JSONDocumentStore` is already a `Sendable` struct with no shared mutable state — making it an `actor`, or giving it `async` methods, would move all of this off main without changing any model.

Start by locating JSONDocumentStore and its listed callers: saveNowPlayingSnapshotNow(), saveHistoryNow(), loadIfNeeded(), and loadAllSavedPlaylists(). Trace the launch, pause, backgrounding, debounced-task, and recorded-play paths; done means persistence encoding and file I/O no longer run on the @MainActor while existing model behavior remains unchanged.

Written by the indexing model from the issue text.