The /api/vault/unlock handler ran `bw` as
`_run_bw(["unlock", req.master_password, "--raw"])`. _run_bw launches it with
`asyncio.create_subprocess_exec(bw_path, *args)`, so the master password became
a process argument — readable by any local user through `ps` and
`/proc/<pid>/cmdline` for the lifetime of the unlock subprocess. The Bitwarden
master password decrypts the entire vault, so this is a serious credential
exposure on any multi-user / shared host (CWE-214).
The sibling /login handler already avoids this by feeding the password on
stdin; unlock was the outlier. Hand the password to `bw` through the
environment instead (`--passwordenv BW_PASSWORD`), mirroring how BW_SESSION is
already passed — `/proc/<pid>/environ` is readable only by the process owner,
not other local users. Add regression tests pinning that the secret reaches
the subprocess env and never appears in argv.
