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/__init__.py

@@ -2,7 +2,17 @@ from app.models.api_key import ApiKey
 from app.models.event import Event
 from app.models.host import Host
 from app.models.notification_channel import NotificationChannel
+from app.models.notification_cooldown import NotificationCooldown
 from app.models.notification_policy import NotificationPolicy
 from app.models.problem import Problem, ProblemEvent
 
-__all__ = ["ApiKey", "Event", "Host", "NotificationChannel", "NotificationPolicy", "Problem", "ProblemEvent"]
+__all__ = [
+    "ApiKey",
+    "Event",
+    "Host",
+    "NotificationChannel",
+    "NotificationCooldown",
+    "NotificationPolicy",
+    "Problem",
+    "ProblemEvent",
+]