fix: create pgadmin4.log with mode 0o600

Override EnhancedRotatingFileHandler._open() so the active log file
and rotated backups are created owner-only on POSIX. The parent
DATA_DIR is already 0o700, so this is defense-in-depth — but the
log file can contain sensitive context (auth events, query
fragments, paths) and should not be world/group readable on its own.

Pre-existing log files keep their current permissions; new installs
and rotated files pick up the tighter mode. Windows uses the default
open path since os.open mode is ignored there.