fix(backup): carve the btrfs snapshot inside its subject, not beside it

The kernel refuses a snapshot whose destination sits on another
filesystem. Placing it at <parent>/.baudolo-<tag> hit that on the most
sensible layout there is: a dedicated disk mounted straight onto
/var/lib/docker, where the parent directory belongs to a different
filesystem and every run failed with EXDEV.

Placing it at <subject>/.baudolo-<tag> makes source and destination the
same filesystem by construction, so the failure cannot occur on any
layout. It also aligns the two backends: the zfs path already resolves
its snapshot inside the subject, at <subject>/.zfs/snapshot/<tag>.

btrfs does not include nested subvolumes in a snapshot, so a leftover
from an interrupted run appears in the next snapshot as an empty
directory rather than recursing, and the copy only ever reads the volume
tree below it.

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

tests/unit/backup/test_app_snapshot.py

@@ -64,7 +64,7 @@ class TestSnapshotBranch(unittest.TestCase):
 
     def test_it_reads_from_the_snapshot_and_not_from_the_live_tree(self) -> None:
         source = drive(present=True)[0]["source"]
-        self.assertTrue(source.startswith("/var/lib/.baudolo-"), source)
+        self.assertTrue(source.startswith("/var/lib/docker/.baudolo-"), source)
 
     def test_it_compares_by_content_against_the_previous_generation(self) -> None:
         self.assertTrue(drive(present=True)[0]["authoritative"])