Move backups to bur-cli

Backups currently live on the same VPS as the database via backup.sh, so a dead disk takes the backups with it. bur-cli is being built for exactly this job (encrypted Postgres backups, local + R2), so this app should use it instead of growing its own offsite logic.

• Once bur can back up a single DB and copy to R2, point it at the album-reviews Postgres container and retire backup.sh
• The Jenkins pre-deploy backup step calls bur instead
• Do one restore drill against a scratch database to prove the path works
• Interim fix while waiting on bur: stop backup.sh exporting every var in .env just to read the two POSTGRES_* ones