* fix(gateway): added MATRIX_ALLOWED_USERS to list of env vars checked by gateway
* fix(gateway): add all missing platform allowlist env vars to startup check
The startup warning for 'No user allowlists configured' was only checking
TELEGRAM, DISCORD, WHATSAPP, SLACK, and SMS — missing SIGNAL, EMAIL,
MATTERMOST, and DINGTALK. Users of those platforms would see a spurious
warning even with their platform-specific allowlist configured.
Now matches the canonical platform_env_map in _is_user_authorized().
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Co-authored-by: SteelPh0enix <wojciech_olech@hotmail.com>
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