Fix backups for JSON-file storage + healthcheck; drop handoff folder

- backup.sh: snapshot all of /data as timestamped tar.gz to local NAS path;
  push to B2 only when a real bucket/key is set (no more nonexistent app.db).
- compose: working CMD-SHELL healthcheck; mount /mnt/user/Backup target.
- remove the accidentally committed moon-household-budget-handoff/ scaffolding.

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

.gitignore

@@ -15,3 +15,6 @@ env/
 .idea/
 .vscode/
 bonnasclaude.md
+
+# handoff/scaffolding folder — not part of the app
+moon-household-budget-handoff/