feat(backup): capture volumes from a filesystem snapshot

Backing up a live volume with rsync copies a moving target: a database
written to mid-copy lands on disk in a state no engine ever committed.
Stopping the container avoids that at the cost of downtime.

A snapshot removes both. `--snapshot {btrfs,zfs}` with `--snapshot-subject`
freezes the docker root once per run, and every volume copy is then read
from that frozen tree while the containers keep serving. A restore of such
a copy is an ordinary crash recovery, which every supported engine performs
on its own at startup.

An unsupported filesystem or an unknown snapshot kind fails loudly rather
than degrading to a live copy, since a silent fallback would return exactly
the torn backup the mode exists to prevent. `--shutdown` is rejected
alongside `--snapshot` instead of being ignored: under a snapshot no
container is ever stopped, so accepting the flag would promise downtime
semantics the run does not deliver.

Copies out of a snapshot skip rsync's --checksum verification. The source
is immutable for the lifetime of the copy, so size-and-mtime cannot race,
and dropping the second full read roughly halves the I/O per volume.

backup/app.py grew past what one module could carry and is split into
layout, policy and dumps along the lines it already had internally.

Tests: unit coverage for the new snapshot, layout, policy, volume and cli
units; e2e cases drive real btrfs, zfs and ext4 filesystems on loop devices
in a privileged container, including a MariaDB that is written to across
the snapshot and must recover from the restored copy without losing a
committed row. CI installs zfs and sets E2E_REQUIRE_FILESYSTEMS so a
missing kernel module fails the build instead of silently skipping a
filesystem.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/baudolo/backup/cli.py

@@ -39,6 +39,16 @@ def parse_args() -> argparse.Namespace:
         help="Backup root directory (e.g. /var/lib/backup/)",
     )
 
+    p.add_argument(
+        "--snapshot",
+        choices=["btrfs", "zfs"],
+        help="Capture every volume from one atomic filesystem snapshot instead of copying the live tree. Containers are not stopped, and the copy is a single pass. Requires --snapshot-subject. Omit to keep the live two-pass copy.",
+    )
+    p.add_argument(
+        "--snapshot-subject",
+        help="Btrfs subvolume or zfs dataset mountpoint holding the docker volumes, e.g. /var/lib/docker. Required with --snapshot.",
+    )
+
     p.add_argument(
         "--database-containers",
         nargs="+",
@@ -79,4 +89,9 @@ def parse_args() -> argparse.Namespace:
             "If a DB dump cannot be produced, baudolo falls back to a file backup."
         ),
     )
-    return p.parse_args()
+    args = p.parse_args()
+    if bool(args.snapshot) != bool(args.snapshot_subject):
+        p.error("--snapshot and --snapshot-subject must be given together")
+    if args.snapshot and args.shutdown:
+        p.error("--shutdown is meaningless with --snapshot: containers are never stopped")
+    return args