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/API.md

@@ -464,6 +464,26 @@ DELETE /api/threads/{thread_id}/uploads/{filename}
 }
 ```
 
+### Thread Cleanup
+
+Remove DeerFlow-managed local thread files under `.deer-flow/threads/{thread_id}` after the LangGraph thread itself has been deleted.
+
+```http
+DELETE /api/threads/{thread_id}
+```
+
+**Response:**
+```json
+{
+  "success": true,
+  "message": "Deleted local thread data for abc123"
+}
+```
+
+**Error behavior:**
+- `422` for invalid thread IDs
+- `500` returns a generic `{"detail": "Failed to delete local thread data."}` response while full exception details stay in server logs
+
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