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/user.py

@@ -35,6 +35,12 @@ class UserCreate(SQLModel):
     role: str = "admin"
 
 
+class UserUpdate(SQLModel):
+    password: Optional[str] = None
+    role: Optional[str] = None
+    is_active: Optional[bool] = None
+
+
 class LoginRequest(SQLModel):
     username: str
     password: str