Phase 4: backups w/ volumes, notifications, settings & users, audit page (0.4.0)

- Backup/restore: per-stack tar.gz incl. named-volume snapshots (helper
  container), upload restore with rename/overwrite/conflict detection.
- Notifications: ntfy/Discord/Slack/Gotify/generic webhooks, per-event
  subscriptions; wired into the update checker and stack lifecycle.
- Settings page: update-check interval, webhook CRUD + test, user management
  (with last-admin safeguards).
- Audit log page (searchable, paginated).
- Mobile-responsive sidebar/layout.

Multi-host agents and remote backup destinations (SFTP/S3) deferred.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/models/__init__.py

@@ -1,7 +1,8 @@
 """SQLModel table models. Importing this package registers all tables."""
 from models.audit import AuditLog
+from models.setting import Setting, Webhook
 from models.stack import Stack
 from models.template import Template
 from models.user import User
 
-__all__ = ["User", "Stack", "AuditLog", "Template"]
+__all__ = ["User", "Stack", "AuditLog", "Template", "Setting", "Webhook"]