fix(backups): enforce retention and clean up stale executions

Add `WithoutOverlapping` middleware to `DatabaseBackupJob` keyed by backup ID
with timeout-based lock expiry to prevent concurrent runs.

Mark long-running backup executions as failed when they exceed the stale
time threshold, and add periodic retention enforcement in
`CleanupInstanceStuffsJob` with cache-based throttling.

Also add float casts for retention max-storage fields on
`ScheduledDatabaseBackup` and comprehensive feature tests covering
overlap middleware, stale detection, casts, and retention behavior.