fix(backups): pin S3 per volume execution and harden deletes

Store s3_storage_id on scheduled volume backup executions so retention
and recovery use the S3 that received the upload. Extract
DeleteScheduledVolumeBackup for UI and resource deletion, and isolate
database backup retention failures so cleanup errors do not fail the job.

app/Models/ScheduledVolumeBackupExecution.php

@@ -9,6 +9,7 @@ class ScheduledVolumeBackupExecution extends BaseModel
     protected $fillable = [
         'uuid',
         'scheduled_volume_backup_id',
+        's3_storage_id',
         'status',
         'message',
         'size',
@@ -40,4 +41,9 @@ class ScheduledVolumeBackupExecution extends BaseModel
     {
         return $this->belongsTo(ScheduledVolumeBackup::class);
     }
+
+    public function s3(): BelongsTo
+    {
+        return $this->belongsTo(S3Storage::class, 's3_storage_id');
+    }
 }