Toggle the standalone save status of an item.
| Current State | Action |
|---|---|
| Not saved at all | Creates a new saved_items record (standalone) |
| Saved standalone (not in collections) | Deletes the saved_items record |
| In collections only | No-op — returns current state |
saved_items record would break collection links.
| Status | Code | Description |
|---|---|---|
| 400 | VALIDATION_ERROR | Missing or invalid parameters |
| 401 | UNAUTHORIZED | Not authenticated |