Add 'vaultik remote nuke', rename Processing→Backing up, bits/sec rates

remote nuke: new subcommand that deletes every snapshot's metadata and
every blob from remote storage, leaving the bucket prefix empty.
Requires --force.

User-facing 'Processing' is now 'Backing up' everywhere it referred to
the chunking/upload phase. Files summary line says 'backed up' instead
of 'processed'.

ui.Speed now formats bytes/sec input as bits/sec output (bit/s, Kbit/s,
Mbit/s, Gbit/s). Network transfer rates are conventionally expressed
in bits — the per-blob heartbeat now matches the per-snapshot summary
line which has always been bits/sec.

internal/vaultik/prune.go

@@ -15,6 +15,31 @@ type PruneOptions struct {
 	JSON  bool
 }
 
+// NukeRemote deletes every snapshot's metadata and every blob from remote
+// storage. After this returns successfully the bucket prefix is empty and
+// the next backup starts from scratch.
+//
+// Refuses to run unless force is true. The caller is responsible for
+// confirming with the user.
+func (v *Vaultik) NukeRemote(force bool) error {
+	if !force {
+		return fmt.Errorf("nuke requires --force (this deletes ALL remote snapshots and blobs)")
+	}
+
+	v.UI.Begin("Removing all snapshot metadata from backup destination store.")
+	if _, err := v.RemoveAllSnapshots(&RemoveOptions{Force: true, Remote: true}); err != nil {
+		return fmt.Errorf("removing all snapshots: %w", err)
+	}
+
+	v.UI.Begin("Removing all blobs from backup destination store.")
+	if err := v.PruneBlobs(&PruneOptions{Force: true}); err != nil {
+		return fmt.Errorf("pruning blobs: %w", err)
+	}
+
+	v.UI.Complete("Backup destination store is now empty.")
+	return nil
+}
+
 // PruneBlobsResult contains the result of a blob prune operation
 type PruneBlobsResult struct {
 	BlobsFound   int   `json:"blobs_found"`