Write rclone uploads under a temporary name and move them into place (closes #266)

The rclone backend wrote each object straight to its key, so killing an
upload to a local or sftp remote left a truncated object there that the
next backup trusted.

On every remote with a server-side move, an object is now written under
a name ending in `.partial` and moved onto its key with rclone's
operations.Move, which removes an object already at the key first;
drive, dropbox and others will not move onto one. Listings skip
`.partial` names. Rclone's own copy also requires the PartialUploads
flag; this does not, because hdfs shows a file while it is written
without setting it. Remotes without a move are written in place.

Model: opus-5-5

README.md

@@ -428,6 +428,16 @@ a local or mounted filesystem. Useful for testing or backing up to a NAS.
 **Rclone** (`rclone://remote/path`): Uses rclone's 70+ supported cloud
 providers. Requires rclone to be configured separately (`rclone config`).
 
+An upload cut off part-way leaves nothing under the object's name on S3, which
+shows an object only once its upload has completed, and on the local filesystem
+backend, which writes a temporary file and renames it into place. Rclone
+remotes with a server-side move (local and sftp among them) are written under a
+temporary name ending in `.partial` and moved into place. Rclone remotes without
+one are written in place: where such a remote shows a file while it is still
+being written, a killed upload can leave a truncated object under its name,
+which a later backup takes for complete. A leftover `.partial` file is ignored
+and can be deleted.
+
 Legacy S3 configuration via `s3.*` fields (endpoint, bucket, prefix, etc.) is
 still supported for backward compatibility. `storage_url` takes precedence if
 both are set.