feat: Group chat and push notifications

- Add group chat tables (chat_groups, chat_group_members, group_messages)
- Add push subscriptions table for web push notifications
- Add group service, controller, routes
- Add push service, controller, routes
- Integrate push notifications with todos, messages, group messages

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

package.json

@@ -33,6 +33,7 @@
     "node-cron": "^4.2.1",
     "pg": "^8.16.3",
     "uuid": "^13.0.0",
+    "web-push": "^3.6.7",
     "xss-clean": "^0.1.4",
     "zod": "^4.1.12"
   },