While data/admin_password exists and a user's password still matches it,
the login sets a session flag (checked once at login, not per request).
ForcePasswordChangeMiddleware then sends that user to the password-change
page for every view except change password, login and logout. Setting a
new password clears the flag and deletes data/admin_password. No schema
change; if ADMIN_PASSWORD was set, there is no file and nothing is forced.
