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/config.py

@@ -70,6 +70,11 @@ class Settings(BaseSettings):
     notify_min_severity: str = "warning"
     notify_channels: str = "telegram,webhook,email"
 
+    # F-NOT-03: cooldown исходящих оповещений (dedup_key / fingerprint)
+    sac_notify_cooldown_enabled: bool = True
+    sac_notify_event_cooldown_sec: int = 90
+    sac_notify_problem_cooldown_sec: int = 300
+
     # Порог «живости» агента
     sac_heartbeat_stale_minutes: int = 780