fix: skip re-extraction if source directory already exists

When reusing a source image (Copy flow), the archive was already
extracted in /mnt/converter/in/. The extraction step failed with
'Destination path already exists' from 7z.

Now extract_if_needed checks if the expected output directory already
contains disk images, and returns it directly without re-extracting.

backend/converter.py

@@ -6,6 +6,7 @@ Runs on the Proxmox host (srv2) where qemu-img, 7z, unzip, tar are available.
 
 from __future__ import annotations
 
+import logging
 import os
 import re
 import shutil
@@ -17,6 +18,8 @@ from typing import Optional
 STAGING_IN = Path("/mnt/converter/in")
 STAGING_OUT = Path("/mnt/converter/out")
 
+logger = logging.getLogger("backend.converter")
+
 DISK_EXTENSIONS = {".vmdk", ".qcow2", ".qcow", ".img", ".raw", ".vhd", ".vhdx"}
 
 ARCHIVE_EXTENSIONS = {".7z", ".zip", ".rar", ".tar.gz", ".tgz", ".tar", ".gz", ".bz2", ".xz"}
@@ -77,6 +80,16 @@ def extract_if_needed(filename: str) -> Path:
             stem = stem[:-len(e)]
             break
 
+    # If already extracted, skip re-extraction
+    expected_dir = STAGING_IN / stem
+    if expected_dir.is_dir() and any(
+        f.suffix.lower() in DISK_EXTENSIONS
+        for f in expected_dir.rglob("*")
+        if f.is_file()
+    ):
+        logger.info("Already extracted: %s", expected_dir)
+        return expected_dir
+
     # Snapshot STAGING_IN before extraction
     before = set(STAGING_IN.iterdir()) if STAGING_IN.exists() else set()