feat: notification cooldown dedup for events and problems (notif-31)

Store last notify time by dedup_key/fingerprint, gate dispatch before
channels; config SAC_NOTIFY_*_COOLDOWN_SEC, migration 008.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/models/notification_cooldown.py

@@ -0,0 +1,16 @@
+from datetime import datetime
+
+from sqlalchemy import DateTime, String
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.database import Base
+
+
+class NotificationCooldown(Base):
+    """Последняя отправка оповещения по ключу (dedup_key / fingerprint)."""
+
+    __tablename__ = "notification_cooldown"
+
+    cooldown_key: Mapped[str] = mapped_column(String(512), primary_key=True)
+    kind: Mapped[str] = mapped_column(String(16))  # event | problem
+    last_notified_at: Mapped[datetime] = mapped_column(DateTime(timezone=True))