Backups, releases, and session security

Protected Safety backups page with administrator-only downloads, file type, timestamp, size, and SHA-256 checksum
Update backups remain outside the public web root under storage/backups/
Configurable inactivity logout, defaulting to 60 minutes
Official update check against skymission.info with direct compatibility selection and automatic checksum prefill
Public SkyMission download center under /download
SkyMission release publisher restricted to installations hosted under `skymission.info`; uploaded packages require a matching SHA-256 and complete manifest/payload validation
Administrator upload for matching WebODM Plugin ZIP and English TXT release notes, with no separate public TXT download
Universal install/update package supports Betas 0.15 through 0.19