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/docs/ARCHITECTURE.md

@@ -31,6 +31,7 @@ This document provides a comprehensive overview of the DeerFlow backend architec
 │  - Thread Mgmt      │ │  - MCP Config       │ │  - React UI         │
 │  - SSE Streaming    │ │  - Skills Mgmt      │ │  - Chat Interface   │
 │  - Checkpointing    │ │  - File Uploads     │ │                     │
+│                     │ │  - Thread Cleanup   │ │                     │
 │                     │ │  - Artifacts        │ │                     │
 └─────────────────────┘ └─────────────────────┘ └─────────────────────┘
           │                       │
@@ -86,7 +87,11 @@ FastAPI application providing REST endpoints for non-agent operations.
 - `mcp.py` - `/api/mcp` - MCP server configuration
 - `skills.py` - `/api/skills` - Skills management
 - `uploads.py` - `/api/threads/{id}/uploads` - File upload
+- `threads.py` - `/api/threads/{id}` - Local DeerFlow thread data cleanup after LangGraph deletion
 - `artifacts.py` - `/api/threads/{id}/artifacts` - Artifact serving
+- `suggestions.py` - `/api/threads/{id}/suggestions` - Follow-up suggestion generation
+
+The web conversation delete flow is now split across both backend surfaces: LangGraph handles `DELETE /api/langgraph/threads/{thread_id}` for thread state, then the Gateway `threads.py` router removes DeerFlow-managed filesystem data via `Paths.delete_thread_dir()`.
 
 ### Agent Architecture
 
@@ -404,6 +409,21 @@ SKILL.md Format:
    - Agent can access via virtual_path
 ```
 
+### Thread Cleanup Flow
+
+```
+1. Client deletes conversation via LangGraph
+   DELETE /api/langgraph/threads/{thread_id}
+
+2. Web UI follows up with Gateway cleanup
+   DELETE /api/threads/{thread_id}
+
+3. Gateway removes local DeerFlow-managed files
+   - Deletes .deer-flow/threads/{thread_id}/ recursively
+   - Missing directories are treated as a no-op
+   - Invalid thread IDs are rejected before filesystem access
+```
+
 ### Configuration Reload
 
 ```