Editor autosave brings back a recipe renamed or deleted in another tab #808

When a recipe is renamed (or deleted) while another editor tab still has it open, that other tab's autosave writes the old file back.

To reproduce:

1. Open the same recipe in the editor in two tabs.
2. In tab A, rename it (Rename → "New Name"), or delete it.
3. In tab B, type anything.

A second later, B's autosave sends PUT /api/recipes/<old path>. That endpoint creates a missing file, so the old name reappears beside the renamed one: two copies, and B's later edits go to the old one. A deleted recipe comes back.

The editor already guards its own rename (renaming in templates/edit.html, whose comment describes exactly this), but not a rename or delete from another tab or device. In-editor rename came in with 2c6e839 (#623); the delete case predates it.