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/app.py

@@ -14,6 +14,7 @@ from app.gateway.routers import (
     models,
     skills,
     suggestions,
+    threads,
     uploads,
 )
 from deerflow.config.app_config import get_app_config
@@ -127,6 +128,10 @@ This gateway provides custom endpoints for models, MCP configuration, skills, an
                 "name": "uploads",
                 "description": "Upload and manage user files for threads",
             },
+            {
+                "name": "threads",
+                "description": "Manage DeerFlow thread-local filesystem data",
+            },
             {
                 "name": "agents",
                 "description": "Create and manage custom agents with per-agent config and prompts",
@@ -167,6 +172,9 @@ This gateway provides custom endpoints for models, MCP configuration, skills, an
     # Uploads API is mounted at /api/threads/{thread_id}/uploads
     app.include_router(uploads.router)
 
+    # Thread cleanup API is mounted at /api/threads/{thread_id}
+    app.include_router(threads.router)
+
     # Agents API is mounted at /api/agents
     app.include_router(agents.router)