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/workers/health_review.py

@@ -1,5 +1,6 @@
 """Daily job: proactive health review for all users with health data."""
 import asyncio
+import json
 import logging
 
 from anthropic import AsyncAnthropic
@@ -48,7 +49,6 @@ async def _review_user(user: User):
             messages=[{"role": "user", "content": f"User health profile:\n{memory_text}"}],
         )
 
-        import json
         try:
             text = response.content[0].text.strip()
             # Extract JSON from response
@@ -71,13 +71,8 @@ async def _review_user(user: User):
                 )
                 await db.commit()
 
+                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),
                 })