save_env_value() used bare open('w') which truncates .env immediately.
A crash or OOM kill between truncation and completed write silently
wipes every credential in the file.
Write now goes to a temp file first, then os.replace() swaps it
atomically. Either the old .env exists or the new one does — never
a truncated half-write. Same pattern used in cron/jobs.py.
Cherry-picked from PR #842 by alireza78a, rebased onto current main
with conflict resolution (_secure_file refactor).
Co-authored-by: alireza78a <alireza78a@users.noreply.github.com>
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