Make remote nuke delete leftover .partial uploads (closes #281)

The file and rclone listings skip an object whose name ends in
`.partial`, the temporary name a `file://` or rclone upload writes
before moving the object into place. `remote nuke` deletes only what
the listings return, so it left the `.partial` objects killed uploads
leave behind and still reported the destination store empty.

Storer gains DeletePartialUploads. The file and rclone backends remove
every `.partial` object under the prefix; S3 has none to remove, since
it shows an object only once its upload completes. `remote nuke` calls
it for `metadata/` and `blobs/` as its last step.

Empty directories under a `file://` destination are still left behind.

Model: opus-5-5

internal/vaultik/nuke_remote_test.go

@@ -10,30 +10,44 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"sneak.berlin/go/vaultik/internal/log"
+	"sneak.berlin/go/vaultik/internal/snapshot"
 )
 
 // TestNukeRemoteLeavesNoFiles checks that remote nuke leaves no file
-// under a file:// destination, including the `.partial` file an upload
-// killed part-way leaves next to where its blob would have been.
+// under a file:// destination, including the `.partial` files uploads
+// killed part-way leave next to a snapshot's metadata and next to where a
+// blob would have been.
 func TestNukeRemoteLeavesNoFiles(t *testing.T) {
 	log.Initialize(log.Config{})
 	t.Parallel()
 
 	ctx := context.Background()
 	storeDir := filepath.Join(t.TempDir(), "store")
-	v, _, _ := backUpToFileDestination(ctx, t, storeDir)
+	v, repos, _ := backUpToFileDestination(ctx, t, storeDir)
 
+	snapshots, err := repos.Snapshots.ListRecent(ctx, listRecentTestLimit)
+	require.NoError(t, err)
+	require.Len(t, snapshots, 1)
+
+	snapshotKey := snapshot.RemoteSnapshotKey(snapshots[0].ID.String())
 	hash := testBlobHashA
-	leftover := filepath.Join(storeDir, "blobs", hash[:2], hash[2:4],
-		hash+"-123456.partial")
-	require.NoError(t, os.MkdirAll(filepath.Dir(leftover), 0o750))
-	require.NoError(t, os.WriteFile(leftover, []byte("half a blob"), 0o600))
+	leftovers := []string{
+		filepath.Join(storeDir, "metadata", snapshotKey,
+			"db.zst.age-123456.partial"),
+		filepath.Join(storeDir, "blobs", hash[:2], hash[2:4],
+			hash+"-123456.partial"),
+	}
+
+	for _, leftover := range leftovers {
+		require.NoError(t, os.MkdirAll(filepath.Dir(leftover), 0o750))
+		require.NoError(t, os.WriteFile(leftover, []byte("half an upload"), 0o600))
+	}
 
 	require.NoError(t, v.NukeRemote(true))
 
 	var files []string
 
-	err := filepath.WalkDir(storeDir,
+	err = filepath.WalkDir(storeDir,
 		func(path string, entry fs.DirEntry, err error) error {
 			if err != nil {
 				return err