stream: expose OS media controls via the Media Session API

Wire the web player into system media controls (MPRIS, Control Center,
lock screen) and keyboard media keys: push track metadata and handle
play/pause/next, with seek neutralized on the live stream.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -32,6 +32,10 @@ simultaneous listeners), not for public broadcasting.
 - **Built-in web player** at `http://localhost:8000/`: now playing (linked to
   its source page), track history, skip button, per-track download, volume
   memory, live auto-reconnect, prefetch progress, and a synthwave look.
+- **OS media controls**: the player exposes current track metadata and
+  play/pause/next through the Media Session API, so it wires into system media
+  controls (MPRIS on Linux, macOS Control Center, Windows, mobile lock screen)
+  and keyboard media keys.
 - **Robust in a container**: Docker healthcheck, graceful shutdown, retries on
   transient HTTP errors.