fix: auto-cleanup staging files to minimize disk usage

Deletes intermediate files as conversion progresses:
- Archive (.7z/.zip) after extraction
- Source VMDK after qemu-img convert
- Local QCOW2 after qm disk import

Reduces peak storage from zip+VMDK+QCOW2 to just the largest
single file — critical for the 94 GB rootfs with 87 GB images.

backend/provisioner.py

@@ -259,11 +259,27 @@ def _process_job(job_id: str, req: JobSubmissionRequest):
                 logger.info("Disk %d: source=%s format=%s size=%.1f GiB",
                             idx, source_path, disk.format, disk.size_gb)
 
+                # Delete archive after extraction to free space
+                if src_path.exists() and src_path != source and src_path.is_file():
+                    try:
+                        src_path.unlink()
+                        logger.info("Deleted archive: %s", src_path)
+                    except Exception:
+                        pass
+
                 # Convert with real-time progress
                 _convert_with_progress(
                     job_id, source_path, output_path, disk.format,
                     start_pct=10 + idx * 10, end_pct=55 + idx * 10,
                 )
+
+                # Delete source VMDK after conversion to free space
+                if source_path.exists() and source_path != src_path:
+                    try:
+                        source_path.unlink()
+                        logger.info("Deleted source disk: %s", source_path)
+                    except Exception:
+                        pass
             else:
                 size_gb = spec.size_gb or 32
                 logger.info("Creating empty disk %d: %d GiB", idx, size_gb)
@@ -378,6 +394,14 @@ def _process_job(job_id: str, req: JobSubmissionRequest):
             _update_job(job_id, progress=75 + idx * 10,
                         message=f"Imported disk {idx + 1}/{len(disk_paths)}")
 
+            # Delete local QCOW2 after import to free space
+            if disk_path.exists():
+                try:
+                    disk_path.unlink()
+                    logger.info("Deleted local QCOW2: %s", disk_path)
+                except Exception:
+                    pass
+
         # ── Add EFI disk if UEFI ──────────────────────────────────
         if final_boot.value == "uefi":
             logger.info("Adding EFI disk")