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/models/__init__.py

@@ -1,3 +1,23 @@
-from app.models.models import AlertDefinition, AuditLog, EmailNotificationSettings, Metric, QueryStat, Target, User
+from app.models.models import (
+    AlertDefinition,
+    AlertNotificationEvent,
+    AuditLog,
+    EmailNotificationSettings,
+    Metric,
+    QueryStat,
+    Target,
+    TargetOwner,
+    User,
+)
 
-__all__ = ["User", "Target", "Metric", "QueryStat", "AuditLog", "AlertDefinition", "EmailNotificationSettings"]
+__all__ = [
+    "User",
+    "Target",
+    "Metric",
+    "QueryStat",
+    "AuditLog",
+    "AlertDefinition",
+    "EmailNotificationSettings",
+    "TargetOwner",
+    "AlertNotificationEvent",
+]