feat: implement proper WebSocket handler with actix-web-actors

- Replace stub websocket_handler with proper actix_web_actors::ws::start()
- Add WsJobActor that subscribes to JobManager broadcast channel
- Add broadcast::Sender/Receiver to JobManager for real-time status updates
- Emit JobStatusEvent on job state changes (create, update, complete, fail)
- Handle subscribe/unsubscribe client messages for per-job filtering
- Add 5-second heartbeat ping/pong for connection keepalive
- Properly compute Sec-WebSocket-Accept header per RFC 6455

src/api/handlers/mod.rs

@@ -15,4 +15,5 @@ pub mod websocket;
 
 // Re-export commonly used types
 pub use packages::{ApiError, ApiResponse};
-pub use websocket::{WsClientMessage, WsServerMessage};
+// WebSocket message types are now in crate::jobs::websocket
+pub use crate::jobs::websocket::{WsClientMessage, WsServerMessage};