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.

app/Models/ScheduledDatabaseBackup.php

@@ -8,6 +8,14 @@ use Illuminate\Database\Eloquent\Relations\MorphTo;
 
 class ScheduledDatabaseBackup extends BaseModel
 {
+    protected function casts(): array
+    {
+        return [
+            'database_backup_retention_max_storage_locally' => 'float',
+            'database_backup_retention_max_storage_s3' => 'float',
+        ];
+    }
+
     protected $fillable = [
         'uuid',
         'team_id',