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.

README.md

@@ -106,6 +106,7 @@ vaultik [--config <path>] restore <snapshot-id> <target-dir> [paths...] [--verif
 vaultik [--config <path>] prune [--force] [--json]
 vaultik [--config <path>] info
 vaultik [--config <path>] remote info [--json]
+vaultik [--config <path>] remote nuke --force
 vaultik [--config <path>] store info
 vaultik [--config <path>] database purge [--force]
 vaultik completion <bash|zsh|fish|powershell>
@@ -225,6 +226,11 @@ recipients, and local database statistics.
 metadata sizes, blob counts, and orphaned blob detection.
 * `--json`: Output as JSON
 
+**`remote nuke`**: Delete every snapshot's metadata and every blob from the
+backup destination store, leaving the bucket prefix empty. Destructive and
+irreversible.
+* `--force`: Required to confirm destruction.
+
 **`store info`**: Display storage backend type and statistics.
 
 **`database purge`**: Delete the local SQLite state database entirely. Remote