fix: prevent websocket backpressure from freezing sessions

Introduce per-route send queues and dedicated sender tasks so terminal output
does not await slow WebSocket clients. Output is buffered up to a bounded
queue; when full, the oldest data is dropped to keep sessions responsive.

Sender tasks enforce a send timeout and close slow/broken sockets, preventing
terminal run loops from stalling indefinitely.

Tests updated/added to verify:
- queued output instead of direct ws.send_bytes
- sender timeout closes sockets
- queue overflow drops oldest
- session close stops sender task

tests/test_ws_sender.py

@@ -0,0 +1,61 @@
+import asyncio
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+from webterm.config import Config
+from webterm.local_server import LocalServer, WS_SEND_TIMEOUT
+
+
+@pytest.mark.asyncio
+async def test_ws_sender_flushes_queue():
+    server = LocalServer(config_path="./", config=Config(apps=[]), host="localhost", port=8080)
+    ws = MagicMock()
+    ws.send_bytes = AsyncMock()
+    ws.closed = False
+    ws.close = AsyncMock()
+
+    queue: asyncio.Queue[bytes | None] = asyncio.Queue()
+    sender_task = asyncio.create_task(server._ws_sender("rk", ws, queue))
+
+    await queue.put(b"hello")
+    await queue.put(b"world")
+    await queue.put(None)
+
+    await sender_task
+    ws.send_bytes.assert_any_await(b"hello")
+    ws.send_bytes.assert_any_await(b"world")
+
+
+@pytest.mark.asyncio
+async def test_ws_sender_timeout_closes():
+    server = LocalServer(config_path="./", config=Config(apps=[]), host="localhost", port=8080)
+    ws = MagicMock()
+    ws.closed = False
+    ws.close = AsyncMock()
+
+    async def slow_send(_data):
+        await asyncio.sleep(WS_SEND_TIMEOUT * 2)
+
+    ws.send_bytes = AsyncMock(side_effect=slow_send)
+
+    queue: asyncio.Queue[bytes | None] = asyncio.Queue()
+    sender_task = asyncio.create_task(server._ws_sender("rk", ws, queue))
+
+    await queue.put(b"slow")
+    await sender_task
+
+    ws.close.assert_awaited_once()
+
+
+@pytest.mark.asyncio
+async def test_enqueue_ws_data_drops_oldest_when_full():
+    server = LocalServer(config_path="./", config=Config(apps=[]), host="localhost", port=8080)
+    queue: asyncio.Queue[bytes | None] = asyncio.Queue(maxsize=1)
+    server._ws_send_queues["rk"] = queue
+
+    queue.put_nowait(b"first")
+    server._enqueue_ws_data("rk", b"second")
+
+    assert queue.qsize() == 1
+    assert await queue.get() == b"second"