Switch task progress to throttled event-driven updates

Replace timer-based _monitor_progress (1s sleep loop) with a
ProgressReporter class that publishes on actual state changes,
throttled to at most 1 publish per 250ms. A background flush
every 2s keeps ETA/elapsed current during quiet periods.

Switch WebSocket forwarder from get_message() polling (1s timeout)
to async pubsub.listen() for instant Redis-to-WebSocket delivery.

Combined latency improvement: ~1.5s average → ~250ms.

api/ws_routes.py

@@ -109,20 +109,18 @@ async def ws_task_progress(websocket: WebSocket) -> None:
 
     async def _forward_pubsub() -> None:
         """Read from Redis pub/sub and forward to the WebSocket."""
-        while True:
-            message = await pubsub.get_message(
-                ignore_subscribe_messages=True, timeout=1.0
-            )
-            if message and message["type"] == "message":
-                try:
-                    data = json.loads(message["data"])
-                except (json.JSONDecodeError, ValueError):
-                    logger.debug("Malformed pubsub message, skipping")
-                    continue
-                try:
-                    await websocket.send_json({"type": "task_update", **data})
-                except Exception:
-                    break
+        async for message in pubsub.listen():
+            if message["type"] != "message":
+                continue
+            try:
+                data = json.loads(message["data"])
+            except (json.JSONDecodeError, ValueError):
+                logger.debug("Malformed pubsub message, skipping")
+                continue
+            try:
+                await websocket.send_json({"type": "task_update", **data})
+            except Exception:
+                break
 
     async def _handle_client_messages() -> None:
         """Read messages from the client (subscribe, ping)."""