Is your feature request related to a problem? Please describe.
Managing longer whitelists as a comma-separated string in a compose file is cumbersome and error-prone (see also #649, where long whitelist strings caused silent truncation/rejection issues).

Describe the solution you'd like
Add a FILE equivalent for OAUTH_WHITELIST (and ideally OAUTH_BLACKLIST) following the same pattern already established for USERS / USERSFILE:

• TINYAUTH_APPS_name_OAUTH_WHITELISTFILE pointing to a file containing one email address per line
• Consistent with the existing --auth.usersfile / TINYAUTH_AUTH_USERSFILE implementation