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/README.md

@@ -36,7 +36,7 @@ DeerFlow is a LangGraph-based AI super agent with sandbox execution, persistent
 
 **Request Routing** (via Nginx):
 - `/api/langgraph/*` → LangGraph Server - agent interactions, threads, streaming
-- `/api/*` (other) → Gateway API - models, MCP, skills, memory, artifacts, uploads
+- `/api/*` (other) → Gateway API - models, MCP, skills, memory, artifacts, uploads, thread-local cleanup
 - `/` (non-API) → Frontend - Next.js web interface
 
 ---
@@ -125,6 +125,7 @@ FastAPI application providing REST endpoints for frontend integration:
 | `GET /api/memory/status` | Combined config + data |
 | `POST /api/threads/{id}/uploads` | Upload files (auto-converts PDF/PPT/Excel/Word to Markdown, rejects directory paths) |
 | `GET /api/threads/{id}/uploads/list` | List uploaded files |
+| `DELETE /api/threads/{id}` | Delete DeerFlow-managed local thread data after LangGraph thread deletion; unexpected failures are logged server-side and return a generic 500 detail |
 | `GET /api/threads/{id}/artifacts/{path}` | Serve generated artifacts |
 
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