Phase 5: Notifications — WebSocket, APScheduler, AI tool, health review

Backend:
- Notification model + Alembic migration
- Notification service: CRUD, mark read, unread count, pending scheduled
- WebSocket manager singleton for real-time push
- WebSocket endpoint /ws/notifications with JWT auth via query param
- APScheduler integration: periodic notification sender (every 60s),
  daily proactive health review job (8 AM)
- AI tool: schedule_notification (immediate or scheduled)
- Health review worker: analyzes user memory via Claude, creates
  ai_generated notifications with WebSocket push

Frontend:
- Notification API client + Zustand store
- WebSocket hook with auto-reconnect (exponential backoff)
- Notification bell in header with unread count badge + dropdown
- Notifications page with type badges, mark read, mark all read
- WebSocket initialized in AppLayout for app-wide real-time updates
- Enabled notifications nav in sidebar
- English + Russian translations

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

frontend/public/locales/en/translation.json

@@ -90,6 +90,19 @@
     "subtitle": "This context is added to all your AI conversations",
     "placeholder": "Add personal information that the AI should know about you..."
   },
+  "notifications": {
+    "title": "Notifications",
+    "no_notifications": "No notifications yet.",
+    "mark_all_read": "Mark all read",
+    "mark_read": "Mark as read",
+    "view_all": "View all notifications",
+    "types": {
+      "reminder": "Reminder",
+      "alert": "Alert",
+      "info": "Info",
+      "ai_generated": "AI Generated"
+    }
+  },
   "documents": {
     "upload": "Upload",
     "drop_or_click": "Drop a file here or click to browse",