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>

src/baudolo/backup/snapshot.py

@@ -41,7 +41,10 @@ def _resolver(subject: str, root: str) -> Callable[[str], str]:
 
 
 def _btrfs(subject: str, name: str, run: Callable[[str], list[str]]) -> tuple[str, str]:
-    target = os.path.join(os.path.dirname(os.path.abspath(subject)), f".{name}")
+    # The snapshot goes inside the subject, never beside it: the kernel rejects
+    # a snapshot whose destination is on another filesystem, which is exactly
+    # what the parent directory is when the subject is a mountpoint of its own.
+    target = os.path.join(os.path.abspath(subject), f".{name}")
     run(f"btrfs subvolume snapshot -r {subject} {target}")
     return target, f"btrfs subvolume delete {target}"