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>

tests/unit/test_backup.py

@@ -1,12 +1,12 @@
 import unittest
 from unittest.mock import patch
 
-from baudolo.backup.app import requires_stop
+from baudolo.backup.policy import requires_stop
 
 
-@patch("baudolo.backup.app.is_swarm_task", return_value=False)
+@patch("baudolo.backup.policy.is_swarm_task", return_value=False)
 class TestRequiresStop(unittest.TestCase):
-    @patch("baudolo.backup.app.get_image_info")
+    @patch("baudolo.backup.policy.get_image_info")
     def test_requires_stop_false_when_all_images_are_whitelisted(
         self, mock_get_image_info, _mock_is_swarm_task
     ):
@@ -18,7 +18,7 @@ class TestRequiresStop(unittest.TestCase):
         whitelist = ["repo/mastodon:v4", "repo/wordpress:latest"]
         self.assertFalse(requires_stop(containers, whitelist))
 
-    @patch("baudolo.backup.app.get_image_info")
+    @patch("baudolo.backup.policy.get_image_info")
     def test_requires_stop_true_when_any_image_is_not_whitelisted(
         self, mock_get_image_info, _mock_is_swarm_task
     ):
@@ -30,7 +30,7 @@ class TestRequiresStop(unittest.TestCase):
         whitelist = ["repo/mastodon:v4", "repo/wordpress:latest"]
         self.assertTrue(requires_stop(containers, whitelist))
 
-    @patch("baudolo.backup.app.get_image_info")
+    @patch("baudolo.backup.policy.get_image_info")
     def test_requires_stop_true_on_substring_only_match(
         self, mock_get_image_info, _mock_is_swarm_task
     ):
@@ -38,7 +38,7 @@ class TestRequiresStop(unittest.TestCase):
         self.assertTrue(requires_stop(["c1"], ["mastodon"]))
         self.assertTrue(requires_stop(["c1"], ["repo/mastodon:v4"]))
 
-    @patch("baudolo.backup.app.get_image_info")
+    @patch("baudolo.backup.policy.get_image_info")
     def test_requires_stop_true_when_whitelist_empty(
         self, mock_get_image_info, _mock_is_swarm_task
     ):
@@ -47,8 +47,8 @@ class TestRequiresStop(unittest.TestCase):
 
 
 class TestRequiresStopSwarm(unittest.TestCase):
-    @patch("baudolo.backup.app.get_image_info")
-    @patch("baudolo.backup.app.is_swarm_task", return_value=True)
+    @patch("baudolo.backup.policy.get_image_info")
+    @patch("baudolo.backup.policy.is_swarm_task", return_value=True)
     def test_swarm_tasks_never_require_stop(
         self, _mock_is_swarm_task, mock_get_image_info
     ):