nodes: a write changes only what it sends

`PUT /api/nodes/{id}` wrote the whole node, and a field left out was taken as
empty: a rename sent without the body erased the text. The browser always sent
it, which was the other half of the trouble. To rename a document it sent the
body it had — the draft for one open here, a copy fetched a moment earlier for
any other — and without the version it had seen, so a save made in another
tab, on the phone or over the mount in between was written over, kept only as
a revision nobody was told about.

Now `name` and `body`, like the rest, change only when they are sent. A rename
sends the name alone, and for a document open in a tile the `updated_at` that
tile last saw, so a rename of a document saved elsewhere since is refused the
way `:w` is, rather than stamping a new version over it that the next autosave
would then write the draft past. Toggling a space's board sends only that.

docs/wiki/API.md

@@ -15,7 +15,7 @@ sealed or not.
 | `DELETE` | `/api/sessions/{id}` | sign another browser out, and lock the vault |
 | `GET`/`POST` | `/api/spaces` | the spaces / create one |
 | `GET`/`POST` | `/api/nodes` | children (`?parent=`) or search (`?q=`, `?space=`; a file matched by its text has `in_text` and the words around the match as `excerpt`) / create, from a template with `template_id` |
-| `GET`/`PUT`/`DELETE` | `/api/nodes/{id}` | read / update / delete into the trash, subtree and all — or for good, for something already in it |
+| `GET`/`PUT`/`DELETE` | `/api/nodes/{id}` | read / update only the fields sent — `name`, `body`, `created_at`, `has_board`; with `base_updated_at`, refused with `409` once it was saved since / delete into the trash, subtree and all — or for good, for something already in it |
 | `GET`/`DELETE` | `/api/trash` | what was deleted, the last first / empty it |
 | `DELETE` | `/api/trash/{id}` | delete one thing in the trash for good |
 | `POST` | `/api/trash/{id}/restore` | back where it was deleted from, or to its space when that folder is gone |