Add target owners and alert notification management.

This commit implements the addition of `target_owners` and `alert_notification_events` tables, enabling management of responsible users for targets. Backend and frontend components are updated to allow viewing, assigning, and notifying target owners about critical alerts via email.

backend/app/api/routes/alerts.py

@@ -20,6 +20,7 @@ from app.services.alerts import (
     validate_alert_sql,
     validate_alert_thresholds,
 )
+from app.services.alert_notifications import process_target_owner_notifications
 from app.services.audit import write_audit_log
 
 router = APIRouter()
@@ -38,7 +39,9 @@ async def list_alert_status(
     user: User = Depends(get_current_user), db: AsyncSession = Depends(get_db)
 ) -> AlertStatusResponse:
     _ = user
-    return await get_alert_status(db, use_cache=True)
+    payload = await get_alert_status(db, use_cache=True)
+    await process_target_owner_notifications(db, payload)
+    return payload
 
 
 @router.get("/definitions", response_model=list[AlertDefinitionOut])