Per review: after copying the legacy install-dir database to the
effective user directory, rename the original to comfyui.db.bak so a
later launch without --user-directory cannot silently fall back to a
diverged copy, while keeping the file around for recovery. Also hoist
the database_default_path import to module scope.
Per review: default --database-url to None and treat a non-None value
as explicit at resolution time. Removes the sys.argv scan and the
database_url_explicit attribute. database_default_path now serves
directly as the legacy copy source. Adds regression tests for the
unchanged no-flag default path and explicit URLs at the legacy
location.