Move config files to /etc/micronas/, fix WebDAV passwd permissions

- /etc/micronas.env      → /etc/micronas/micronas.env
- /etc/micronas-webdav.passwd → /etc/micronas/webdav.passwd
- _save_credentials creates /etc/micronas/ if missing
- _write_webdav_conf: chown root:www-data on passwd file so Apache can read it
- micronas.service EnvironmentFile updated
- README: credentials now set via /setup, config dir step added

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

micronas.service

@@ -7,7 +7,7 @@ Wants=smbd.service nfs-server.service
 Type=simple
 User=root
 WorkingDirectory=/opt/micronas
-EnvironmentFile=/etc/micronas.env
+EnvironmentFile=/etc/micronas/micronas.env
 ExecStart=/opt/micronas/venv/bin/gunicorn -w 1 -b 0.0.0.0:5000 app:app
 Restart=on-failure
 RestartSec=5