Keep unreferenced blobs newer than prune_grace_period (closes #387)

A backup uploads its blobs before the manifest that lists them, and the
process lock covers one user on one machine, so a prune from another host
or user sharing the destination deleted a running backup's blobs.

Prune now deletes an unreferenced blob only once its listed time is older
than the new config key prune_grace_period (default 24h; a negative value
is refused). A newer blob, or one listed with no time, is kept and counted
(blobs_too_new). The rclone backend takes each object's time from the
listing (use_server_modtime).

Decision: a blob a backup reuses from an earlier run that stopped before
its manifest keeps its old time and is not protected; the docs and config
comments say so.

Model: opus-5-5

internal/cli/entry_prune_quiet_test.go

@@ -5,6 +5,7 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -14,8 +15,8 @@ import (
 const flagCron = "--cron"
 
 // plantUnreferencedBlob writes a blob that no snapshot references to the
-// destination store at storeDir, so that a prune deletes it. Returns its
-// path.
+// destination store at storeDir, dated long before any prune grace period,
+// so that a prune deletes it. Returns its path.
 func plantUnreferencedBlob(t *testing.T, storeDir string) string {
 	t.Helper()
 
@@ -25,6 +26,9 @@ func plantUnreferencedBlob(t *testing.T, storeDir string) string {
 	require.NoError(t, os.MkdirAll(filepath.Dir(path), 0o750))
 	require.NoError(t, os.WriteFile(path, []byte("unreferenced"), 0o600))
 
+	longAgo := time.Unix(0, 0)
+	require.NoError(t, os.Chtimes(path, longAgo, longAgo))
+
 	return path
 }