fix: broadcast WebSocket invalidation on all Message creation paths

[Novanglus96]

Summary

• Adds a post_save signal on the Message model in administration/signals.py
• Signal calls broadcast_invalidate(["messages"]) with the correct group (user_<pk> for user-scoped messages, global for broadcast messages)
• Wires the signal in AdministrationConfig.ready() so it loads on app startup
• Previously, WebSocket invalidation was only triggered via transactions/tasks.py's create_message() helper — messages created via Django shell, admin, or any other path did not push updates to the frontend

Test plan

• Create a Message via Django admin or shell — it should appear in the inbox immediately without a page refresh
• Create a user-scoped Message — only that user's inbox should update
• Create a global Message (user=None) — all connected clients should receive the invalidation

🤖 Generated with Claude Code

[Novanglus96]

🎉 This PR is included in version 1.6.1-alpha.2 🎉

The release is available on GitHub release

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[Novanglus96]

🎉 This PR is included in version 1.6.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀