feat(agent-office): YouTubeResearchAgent + 스케줄러 + /youtube/research API

- db.py: youtube_research_jobs 테이블 추가 + CRUD 3종 (add/update/get_latest)
- agents/youtube.py: YouTubeResearchAgent 신규 구현 (on_schedule/on_command/on_approval/_run_research/send_weekly_report)
- agents/__init__.py: YouTubeResearchAgent 등록
- scheduler.py: youtube_research(매일 09:00) + youtube_weekly_report(월 08:00) cron 추가
- main.py: POST /api/agent-office/youtube/research + GET /api/agent-office/youtube/research/status 엔드포인트 추가

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

agent-office/app/agents/__init__.py

@@ -3,6 +3,7 @@ from .music import MusicAgent
 from .blog import BlogAgent
 from .realestate import RealestateAgent
 from .lotto import LottoAgent
+from .youtube import YouTubeResearchAgent
 
 AGENT_REGISTRY = {}
 
@@ -12,6 +13,7 @@ def init_agents():
     AGENT_REGISTRY["blog"] = BlogAgent()
     AGENT_REGISTRY["realestate"] = RealestateAgent()
     AGENT_REGISTRY["lotto"] = LottoAgent()
+    AGENT_REGISTRY["youtube"] = YouTubeResearchAgent()
 
 def get_agent(agent_id: str):
     return AGENT_REGISTRY.get(agent_id)