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/suggestions.py

@@ -5,7 +5,6 @@ from fastapi import APIRouter, Request
 from langchain_core.messages import HumanMessage, SystemMessage
 from pydantic import BaseModel, Field
 
-from app.gateway.authz import require_permission
 from deerflow.models import create_chat_model
 
 logger = logging.getLogger(__name__)
@@ -99,7 +98,6 @@ def _format_conversation(messages: list[SuggestionMessage]) -> str:
     summary="Generate Follow-up Questions",
     description="Generate short follow-up questions a user might ask next, based on recent conversation context.",
 )
-@require_permission("threads", "read", owner_check=True)
 async def generate_suggestions(thread_id: str, body: SuggestionsRequest, request: Request) -> SuggestionsResponse:
     if not body.messages:
         return SuggestionsResponse(suggestions=[])