fix: harden web terminal paste and reconnect

- Queue bounded ordered terminal input so acknowledgements remain responsive
- Prevent stale sockets and retries from replacing healthy connections
- Preserve desktop scrollback behavior and add terminal regression coverage

Quality gates: frontend tests (91 passed), typecheck, lint, build, Python compilation, LSP diagnostics. Backend pytest skipped by user request.

apps/api/src/api/system/terminal.py

@@ -3,6 +3,8 @@
 import asyncio
 import json
 import logging
+from asyncio import QueueFull
+from json import JSONDecodeError
 import uuid
 from contextlib import suppress
 
@@ -31,6 +33,9 @@ from src.services.terminal.terminal_manager import (
 router = APIRouter()
 logger = logging.getLogger(__name__)
 
+MAX_PENDING_INPUT_MESSAGES = 64
+MAX_TERMINAL_INPUT_BYTES = 1024 * 1024
+
 
 class SessionRef:
     """Mutable reference to a terminal session, allowing updates during reset."""
@@ -320,8 +325,38 @@ async def _handle_terminal_websocket(
                 )
 
 
+async def _input_write_loop(input_queue: asyncio.Queue[tuple[object, bytes]]) -> None:
+    """Serialize PTY writes without blocking terminal control messages."""
+    while True:
+        session, data = await input_queue.get()
+        try:
+            await session.write_input(data)  # type: ignore[attr-defined]
+        except Exception:
+            logger.debug("Terminal input write failed", exc_info=True)
+        finally:
+            input_queue.task_done()
+
+
+def _queue_terminal_input(
+    input_queue: asyncio.Queue[tuple[object, bytes]], session: object, data: bytes
+) -> bool:
+    """Queue bounded terminal input without blocking control-message processing."""
+    if len(data) > MAX_TERMINAL_INPUT_BYTES:
+        return False
+    try:
+        input_queue.put_nowait((session, data))
+    except QueueFull:
+        return False
+    return True
+
+
 async def _write_loop(session_ref: SessionRef, websocket, instance_id: str) -> None:
-    """Read input from WebSocket and send to container."""
+    """Receive terminal messages while a dedicated worker serializes PTY input."""
+    input_queue: asyncio.Queue[tuple[object, bytes]] = asyncio.Queue(
+        maxsize=MAX_PENDING_INPUT_MESSAGES
+    )
+    input_writer = asyncio.create_task(_input_write_loop(input_queue))
+
     try:
         while True:
             session = session_ref.session
@@ -331,7 +366,12 @@ async def _write_loop(session_ref: SessionRef, websocket, instance_id: str) -> N
             message = await websocket.receive()
             if message["type"] == "websocket.receive":
                 if "bytes" in message:
-                    await session.write_input(message["bytes"])
+                    if not _queue_terminal_input(
+                        input_queue, session, message["bytes"]
+                    ):
+                        logger.warning("Terminal input buffer exceeded for %s", instance_id)
+                        await websocket.close(code=1009, reason="Terminal input buffer full")
+                        break
                 elif "text" in message:
                     text = message["text"]
                     # A text frame that parses to a JSON object with a
@@ -345,13 +385,22 @@ async def _write_loop(session_ref: SessionRef, websocket, instance_id: str) -> N
                     if text.startswith("{"):
                         try:
                             parsed = json.loads(text)
-                        except json.JSONDecodeError:
+                        except JSONDecodeError:
                             parsed = None
                         if isinstance(parsed, dict) and "type" in parsed:
                             ctrl = parsed
 
                     if ctrl is None:
-                        await session.write_input(text.encode("utf-8"))
+                        if not _queue_terminal_input(
+                            input_queue, session, text.encode("utf-8")
+                        ):
+                            logger.warning(
+                                "Terminal input buffer exceeded for %s", instance_id
+                            )
+                            await websocket.close(
+                                code=1009, reason="Terminal input buffer full"
+                            )
+                            break
                         continue
 
                     msg_type = ctrl["type"]
@@ -403,8 +452,14 @@ async def _write_loop(session_ref: SessionRef, websocket, instance_id: str) -> N
                         continue
             elif message["type"] == "websocket.disconnect":
                 break
-    except Exception:
+    except WebSocketDisconnect:
         pass
+    except (RuntimeError, TypeError, ValueError) as exc:
+        logger.debug("Terminal WebSocket receive loop ended: %s", exc)
+    finally:
+        input_writer.cancel()
+        with suppress(asyncio.CancelledError):
+            await input_writer
 
 
 async def _heartbeat_loop(websocket: WebSocket) -> None: