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/core/utils.py

@@ -44,3 +44,81 @@ def count_log_errors(log_path: str) -> int:
             return sum(1 for line in f if "[ERROR]" in line)
     except FileNotFoundError:
         return 0
+
+
+# ─── CalDAV 헬퍼 ───
+
+
+def create_caldav_todo(
+    caldav_url: str,
+    username: str,
+    password: str,
+    title: str,
+    description: str = "",
+    due_days: int = 7,
+) -> str | None:
+    """Synology Calendar에 VTODO 생성, UID 반환"""
+    import uuid
+    from datetime import datetime, timedelta, timezone
+
+    import caldav
+
+    try:
+        client = caldav.DAVClient(url=caldav_url, username=username, password=password)
+        principal = client.principal()
+        calendars = principal.calendars()
+        if not calendars:
+            return None
+
+        calendar = calendars[0]
+        uid = str(uuid.uuid4())
+        due = datetime.now(timezone.utc) + timedelta(days=due_days)
+        due_str = due.strftime("%Y%m%dT%H%M%SZ")
+
+        vtodo = f"""BEGIN:VCALENDAR
+VERSION:2.0
+BEGIN:VTODO
+UID:{uid}
+SUMMARY:{title}
+DESCRIPTION:{description}
+DUE:{due_str}
+STATUS:NEEDS-ACTION
+PRIORITY:5
+END:VTODO
+END:VCALENDAR"""
+
+        calendar.save_event(vtodo)
+        return uid
+    except Exception as e:
+        logging.getLogger("caldav").error(f"CalDAV VTODO 생성 실패: {e}")
+        return None
+
+
+# ─── SMTP 헬퍼 ───
+
+
+def send_smtp_email(
+    host: str,
+    port: int,
+    username: str,
+    password: str,
+    subject: str,
+    body: str,
+    to_addr: str | None = None,
+):
+    """Synology MailPlus SMTP로 이메일 발송"""
+    import smtplib
+    from email.mime.text import MIMEText
+
+    to_addr = to_addr or username
+    msg = MIMEText(body, "plain", "utf-8")
+    msg["Subject"] = subject
+    msg["From"] = username
+    msg["To"] = to_addr
+
+    try:
+        with smtplib.SMTP_SSL(host, port, timeout=30) as server:
+            server.login(username, password)
+            server.send_message(msg)
+    except Exception as e:
+        logging.getLogger("smtp").error(f"SMTP 발송 실패: {e}")