Save downloaded covers as folder.jpg (Jellyfin standard), PNG→JPEG on download

- download_cover() now writes folder.jpg instead of _cover_download{ext}
- PNG responses are converted to JPEG via PIL during download (avoids PNG
  in the album directory entirely)
- find_local_cover() priority: folder > front > cover > album (folder.jpg
  is now the canonical name for both downloaded and manually placed covers)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cover_handler.py

@@ -47,7 +47,7 @@ def _image_ok(path: Path) -> bool:
 
 
 def find_local_cover(image_files: List[Path]) -> Optional[Path]:
-    priority = ("front", "folder", "cover", "album")
+    priority = ("folder", "front", "cover", "album")
     # Sort by priority keyword, then size descending
     def key(p: Path):
         name = p.name.lower()
@@ -80,20 +80,27 @@ def download_cover(release_mbid: Optional[str], dest_dir: Path) -> Optional[Path
     url = _mb_cover_url(release_mbid)
     if not url:
         return None
+    dest = dest_dir / "folder.jpg"
     try:
         r = requests.get(url, timeout=15)
-        if r.status_code == 200:
-            ext = ".jpg"
+        if r.status_code != 200:
+            return None
         ct = r.headers.get("content-type", "")
-            if "png" in ct:
-                ext = ".png"
-            dest = dest_dir / f"_cover_download{ext}"
+        if "png" in ct and HAS_PIL:
+            # PNG → JPEG konvertieren
+            import io
+            with Image.open(io.BytesIO(r.content)) as img:
+                buf = io.BytesIO()
+                img.convert("RGB").save(buf, format="JPEG", quality=92)
+                dest.write_bytes(buf.getvalue())
+        else:
             dest.write_bytes(r.content)
         if _image_ok(dest):
             return dest
         dest.unlink(missing_ok=True)
     except Exception as e:
         print(f"  ⚠️ Cover-Download-Fehler: {e}", file=sys.stderr)
+        dest.unlink(missing_ok=True)
     return None