feat: implement Phase 3 automation workers

- Add automation_state table for incremental sync (last UID, last check)
- Add law_monitor worker: 국가법령정보센터 API → NAS/DB/CalDAV VTODO
  (LAW_OC 승인 대기 중, 코드 완성)
- Add mailplus_archive worker: IMAP(993) → .eml NAS save + DB + SMTP
  notification (imaplib via asyncio.to_thread, timeout=30)
- Add daily_digest worker: PostgreSQL/pipeline stats → Markdown + SMTP
  (documents, law changes, email, queue errors, inbox backlog)
- Add CalDAV VTODO helper and SMTP email helper to core/utils.py
- Wire 3 cron jobs in APScheduler (law@07:00, mail@07:00+18:00,
  digest@20:00) with timezone=Asia/Seoul

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/models/automation.py

@@ -0,0 +1,20 @@
+"""automation_state 테이블 ORM — 자동화 워커 증분 동기화 상태"""
+
+from datetime import datetime
+
+from sqlalchemy import BigInteger, DateTime, String, Text
+from sqlalchemy.orm import Mapped, mapped_column
+
+from core.database import Base
+
+
+class AutomationState(Base):
+    __tablename__ = "automation_state"
+
+    id: Mapped[int] = mapped_column(BigInteger, primary_key=True)
+    job_name: Mapped[str] = mapped_column(String(50), unique=True, nullable=False)
+    last_check_value: Mapped[str | None] = mapped_column(Text)
+    last_run_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=datetime.now, onupdate=datetime.now
+    )