refactor(routers): reorganize routers with new langgraph/ subdirectory

Restructure app/gateway/routers/:
- Add langgraph/ subdirectory for LangGraph-related endpoints:
  - threads.py - thread management
  - runs.py - run execution and streaming
  - feedback.py - feedback endpoints
  - suggestions.py - follow-up suggestions

Remove old standalone routers:
- threads.py → langgraph/threads.py
- thread_runs.py → langgraph/runs.py
- runs.py (stateless) → langgraph/runs.py
- feedback.py → langgraph/feedback.py

Update existing routers:
- memory.py, uploads.py, artifacts.py, suggestions.py

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/gateway/routers/artifacts.py

@@ -7,7 +7,6 @@ from urllib.parse import quote
 from fastapi import APIRouter, HTTPException, Request
 from fastapi.responses import FileResponse, PlainTextResponse, Response
 
-from app.gateway.authz import require_permission
 from app.gateway.path_utils import resolve_thread_virtual_path
 
 logger = logging.getLogger(__name__)
@@ -82,7 +81,6 @@ def _extract_file_from_skill_archive(zip_path: Path, internal_path: str) -> byte
     summary="Get Artifact File",
     description="Retrieve an artifact file generated by the AI agent. Text and binary files can be viewed inline, while active web content is always downloaded.",
 )
-@require_permission("threads", "read", owner_check=True)
 async def get_artifact(thread_id: str, path: str, request: Request, download: bool = False) -> Response:
     """Get an artifact file by its path.