fix(backup): let the stopped copy replace a failed live pre-copy

A volume whose containers must stop was copied twice: live first, then
with the containers stopped and --checksum. A live writer that truncates
a file under rsync (read errors mapping ...: No data available (61),
exit 23) aborted the run in the first pass, before the containers were
stopped, although the second pass rewrites every differing file anyway.
Seen in infinito-nexus CI on kix_kix_shared/objecticons.

The live copy now only warns when a stopped copy follows. Without a stop
it stays the only copy and still fails the run, and a failing stopped
copy still fails it.

The e2e test drives a writer that shrinks files mid-read; without the
fix it reproduces the CI failure verbatim.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/e2e/helpers/fixtures.py

@@ -25,7 +25,7 @@ def backup_run(
     images_no_stop_required: list[str],
     images_no_backup_required: list[str] | None = None,
     only_sql: bool = False,
-) -> None:
+) -> subprocess.CompletedProcess:
     cmd = [
         "baudolo",
         "--compose-dir",
@@ -49,7 +49,7 @@ def backup_run(
         cmd += ["--only-sql"]
 
     try:
-        run(cmd, capture=True, check=True)
+        return run(cmd, capture=True, check=True)
     except subprocess.CalledProcessError as e:
         print(">>> baudolo failed (exit code:", e.returncode, ")")
         if e.stdout: