refactor: improve admin notification service and add tests

- Minor improvements to admin notification service
- Add unit tests for admin notification service

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/admin_notification_service.py

@@ -12,7 +12,7 @@ import logging
 from datetime import datetime, timedelta
 from typing import Any
 
-from sqlalchemy import and_, func, or_
+from sqlalchemy import and_, case, func, or_
 from sqlalchemy.orm import Session
 
 from models.database.admin import AdminNotification, PlatformAlert
@@ -176,7 +176,7 @@ class AdminNotificationService:
         )
 
         # Get paginated results ordered by priority and date
-        priority_order = func.case(
+        priority_order = case(
             (AdminNotification.priority == "critical", 1),
             (AdminNotification.priority == "high", 2),
             (AdminNotification.priority == "normal", 3),
@@ -211,7 +211,7 @@ class AdminNotificationService:
         limit: int = 5,
     ) -> list[AdminNotification]:
         """Get recent unread notifications for header dropdown."""
-        priority_order = func.case(
+        priority_order = case(
             (AdminNotification.priority == "critical", 1),
             (AdminNotification.priority == "high", 2),
             (AdminNotification.priority == "normal", 3),
@@ -554,7 +554,7 @@ class PlatformAlertService:
         )
 
         # Get paginated results
-        severity_order = func.case(
+        severity_order = case(
             (PlatformAlert.severity == "critical", 1),
             (PlatformAlert.severity == "error", 2),
             (PlatformAlert.severity == "warning", 3),