Sequence diagram: Stream URL fetch failure

sequenceDiagram
    participant User
    participant Shell as Svelte Shell
    participant Engine as Playback Engine
    participant HTTP as HTTP Client (reqwest)

    User->>Shell: Press play
    Shell->>Engine: invoke("play_track", source)
    Engine->>Engine: Resolve stream URL (via Plugin Runtime)
    Engine-->>Engine: Got StreamInfo { url, expires_at }

    Note over Engine: expires_at is null or in the future — proceed

    Engine->>HTTP: GET url
    HTTP-->>Engine: 404 Not Found

    Engine-->>Shell: emit("playback_error", { error: "Stream unavailable" })
    Engine->>Engine: Auto-advance queue
    Engine-->>Shell: emit("track_changed", next_track)

    Shell-->>User: Show toast "Stream unavailable, skipping"

Expired URL variant

sequenceDiagram
    participant Engine as Playback Engine
    participant RT as Plugin Runtime

    Engine->>Engine: Check StreamInfo.expires_at
    Note over Engine: expires_at is in the past — URL is stale
    Engine->>RT: stream(plugin_id, content_id) — re-resolve
    RT-->>Engine: StreamInfo { url: new_url, expires_at: future }
    Engine->>Engine: Fetch new_url