feat(qbzd): unified event bus + SSE endpoint (Phase 3)

Playback event loop polls QbzCore player state with adaptive timing
(250ms playing, 1s paused, 5s idle) and broadcasts PlaybackSnapshot
events to the event bus.

New SSE endpoint at /api/events streams all DaemonEvents to clients.
Broadcast channel bounded to 256 events with lagged client handling.

cc

crates/qbzd/Cargo.toml

@@ -26,7 +26,9 @@ tower-http = { version = "0.6", features = ["cors"] }
 
 # Async
 tokio = { workspace = true }
+tokio-stream = { workspace = true }
 async-trait = { workspace = true }
+futures-util = { workspace = true }
 
 # Serialization
 serde = { workspace = true }