Fix Phase 5 review issues: mark_all_read, datetime, WS payload, logging

Review fixes:
- mark_all_read now filters status IN (sent, delivered), preventing
  pending notifications from being incorrectly marked as read
- schedule_notification tool: force UTC on naive datetimes, reject
  past dates (send immediately instead)
- WebSocket notification payload now includes all fields matching
  NotificationResponse (user_id, channel, status, scheduled_at, etc.)
- Centralized _serialize_notification helper in notification_sender
- notification_sender: per-notification error handling with rollback
- health_review: moved import json to module top level

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/ai_service.py

@@ -129,10 +129,14 @@ async def _execute_tool(
         return json.dumps({"entries": items, "count": len(items)})
 
     elif tool_name == "schedule_notification":
-        from datetime import datetime
+        from datetime import datetime, timezone as tz
         scheduled_at = None
         if tool_input.get("scheduled_at"):
             scheduled_at = datetime.fromisoformat(tool_input["scheduled_at"])
+            if scheduled_at.tzinfo is None:
+                scheduled_at = scheduled_at.replace(tzinfo=tz.utc)
+            if scheduled_at <= datetime.now(tz.utc):
+                scheduled_at = None  # Past date → send immediately
 
         notif = await create_notification(
             db, user_id,
@@ -145,15 +149,10 @@ async def _execute_tool(
 
         # Push immediately if not scheduled
         if not scheduled_at:
+            from app.workers.notification_sender import _serialize_notification
             await manager.send_to_user(user_id, {
                 "type": "new_notification",
-                "notification": {
-                    "id": str(notif.id),
-                    "title": notif.title,
-                    "body": notif.body,
-                    "type": notif.type,
-                    "created_at": notif.created_at.isoformat(),
-                },
+                "notification": _serialize_notification(notif),
             })
 
         return json.dumps({