feat(isolation): wire user_id through all Paths and memory callsites

Pass user_id=get_effective_user_id() at every callsite that invokes
Paths methods or memory functions, enabling per-user filesystem isolation
throughout the harness and app layers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/gateway/routers/uploads.py

@@ -9,6 +9,7 @@ from pydantic import BaseModel
 
 from app.gateway.authz import require_permission
 from deerflow.config.paths import get_paths
+from deerflow.runtime.user_context import get_effective_user_id
 from deerflow.sandbox.sandbox_provider import get_sandbox_provider
 from deerflow.uploads.manager import (
     PathTraversalError,
@@ -69,7 +70,7 @@ async def upload_files(
         uploads_dir = ensure_uploads_dir(thread_id)
     except ValueError as e:
         raise HTTPException(status_code=400, detail=str(e))
-    sandbox_uploads = get_paths().sandbox_uploads_dir(thread_id)
+    sandbox_uploads = get_paths().sandbox_uploads_dir(thread_id, user_id=get_effective_user_id())
     uploaded_files = []
 
     sandbox_provider = get_sandbox_provider()
@@ -147,7 +148,7 @@ async def list_uploaded_files(thread_id: str, request: Request) -> dict:
     enrich_file_listing(result, thread_id)
 
     # Gateway additionally includes the sandbox-relative path.
-    sandbox_uploads = get_paths().sandbox_uploads_dir(thread_id)
+    sandbox_uploads = get_paths().sandbox_uploads_dir(thread_id, user_id=get_effective_user_id())
     for f in result["files"]:
         f["path"] = str(sandbox_uploads / f["filename"])