refactor(backup)!: run argv lists, never a shell

The backup built command strings and handed them to shell=True, so four interpolated values per dump - user, password, container, database - were each a way out of the command. validate_database covered one of them since the previous commit; now there is nothing to cover: every command is an argv list, and a value can only ever be an argument.

execute_to_file absorbs the atomic dump write. The shell redirect into <file>.tmp and the separate mv process become a Python file handle and os.replace, and a failing dump deletes its partial file instead of leaving it. PGPASSWORD moves out of the command string into the child's environment, where a process listing does not show it.

docker exec is built in one place, docker_exec_argv; db.py's three hand-built copies and the probe use it. The dead docker_volume_exists goes - never called, and the restore side owns the living twin. The rsync quoting in --link-dest falls away: inside an argv it would have become part of the path.

The snapshot module's injected runner changes type with it, which the three e2e drivers implement - the first conversion missed them, btrfs ran with no arguments, and the e2e caught it. Marked breaking for that contract: any external runner injected into volume_snapshot must now accept a list.

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

src/baudolo/backup/layout.py

@@ -11,7 +11,7 @@ from .shell import BackupException, execute_shell_command
 
 
 def get_machine_id() -> str:
-    return execute_shell_command("sha256sum /etc/machine-id")[0][0:64]
+    return execute_shell_command(["sha256sum", "/etc/machine-id"])[0][0:64]
 
 
 def stamp_directory(version_dir: str) -> None: