Normalize all cover art to folder.jpg (Jellyfin standard)

- Add normalize_cover_to_folder_jpg(): renames/converts any local cover
  (Front.jpg, front.webp, cover.jpg, …) to folder.jpg in-place; WebP/PNG
  are converted to JPEG via PIL
- resolve_cover() now calls normalize_cover_to_folder_jpg() automatically
  after finding a local cover, so future enrichment runs always produce folder.jpg
- One-time batch: 38 existing library covers renamed/converted to folder.jpg

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

cover_handler.py

@@ -48,7 +48,6 @@ def _image_ok(path: Path) -> bool:
 
 def find_local_cover(image_files: List[Path]) -> Optional[Path]:
     priority = ("folder", "front", "cover", "album")
-    # Sort by priority keyword, then size descending
     def key(p: Path):
         name = p.name.lower()
         score = next((i for i, kw in enumerate(priority) if kw in name), len(priority))
@@ -61,6 +60,41 @@ def find_local_cover(image_files: List[Path]) -> Optional[Path]:
     return None
 
 
+def normalize_cover_to_folder_jpg(cover_path: Path) -> Path:
+    """
+    Stellt sicher dass das Cover als folder.jpg (JPEG) im Album-Verzeichnis liegt.
+    - Ist es bereits folder.jpg → unverändert zurückgeben.
+    - Ist es eine andere JPEG → umbenennen.
+    - Ist es WebP oder PNG → zu JPEG konvertieren, Original löschen.
+    Gibt den Pfad zur folder.jpg zurück.
+    """
+    dest = cover_path.parent / "folder.jpg"
+    if cover_path.resolve() == dest.resolve():
+        return dest
+
+    suffix = cover_path.suffix.lower()
+    try:
+        if suffix in (".jpg", ".jpeg"):
+            cover_path.rename(dest)
+        elif HAS_PIL:
+            import io
+            with cover_path.open("rb") as f:
+                raw = f.read()
+            with Image.open(io.BytesIO(raw)) as img:
+                buf = io.BytesIO()
+                img.convert("RGB").save(buf, format="JPEG", quality=92)
+                dest.write_bytes(buf.getvalue())
+            cover_path.unlink()
+        else:
+            # PIL nicht verfügbar: einfach umbenennen, auch wenn es kein JPEG ist
+            cover_path.rename(dest)
+        print(f"  🖼️  Cover normalisiert → folder.jpg ({cover_path.name})")
+    except Exception as e:
+        print(f"  ⚠️ Cover-Normalisierung fehlgeschlagen: {e}", file=sys.stderr)
+        return cover_path
+    return dest
+
+
 def _mb_cover_url(release_mbid: str) -> Optional[str]:
     url = f"https://coverartarchive.org/release/{release_mbid}/front"
     if not HAS_REQUESTS:
@@ -195,6 +229,7 @@ def resolve_cover(
     """Returns (cover_path, source_label)."""
     local = find_local_cover(image_files)
     if local:
+        local = normalize_cover_to_folder_jpg(local)
         return local, "local"
 
     if release_mbid: