fix(threads): clean up local thread data after thread deletion (#1262)

* fix(threads): clean up local thread data after thread deletion

Delete DeerFlow-managed thread directories after the web UI removes a LangGraph thread.
This keeps local thread data in sync with conversation deletion and adds regression coverage for the cleanup flow.

* fix(threads): address thread cleanup review feedback

Encode thread cleanup URLs in the web client, keep cache updates explicit when no thread search data is cached, and return a generic 500 response from the cleanup endpoint while documenting the sanitized error behavior.

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/app/gateway/routers/threads.py

@@ -0,0 +1,41 @@
+import logging
+
+from fastapi import APIRouter, HTTPException
+from pydantic import BaseModel
+
+from deerflow.config.paths import Paths, get_paths
+
+logger = logging.getLogger(__name__)
+router = APIRouter(prefix="/api/threads", tags=["threads"])
+
+
+class ThreadDeleteResponse(BaseModel):
+    """Response model for thread cleanup."""
+
+    success: bool
+    message: str
+
+
+def _delete_thread_data(thread_id: str, paths: Paths | None = None) -> ThreadDeleteResponse:
+    """Delete local persisted filesystem data for a thread."""
+    path_manager = paths or get_paths()
+    try:
+        path_manager.delete_thread_dir(thread_id)
+    except ValueError as exc:
+        raise HTTPException(status_code=422, detail=str(exc)) from exc
+    except Exception as exc:
+        logger.exception("Failed to delete thread data for %s", thread_id)
+        raise HTTPException(status_code=500, detail="Failed to delete local thread data.") from exc
+
+    logger.info("Deleted local thread data for %s", thread_id)
+    return ThreadDeleteResponse(success=True, message=f"Deleted local thread data for {thread_id}")
+
+
+@router.delete("/{thread_id}", response_model=ThreadDeleteResponse)
+async def delete_thread_data(thread_id: str) -> ThreadDeleteResponse:
+    """Delete local persisted filesystem data for a thread.
+
+    This endpoint only cleans DeerFlow-managed thread directories. LangGraph
+    thread state deletion remains handled by the LangGraph API.
+    """
+    return _delete_thread_data(thread_id)