Add Discogs as cover source fallback after MusicBrainz

- _discogs_cover_url(): searches Discogs database/search API by artist+album,
  returns primary image URL; uses DISCOGS_TOKEN if set, else anonymous
- download_discogs_cover(): downloads and saves as folder.jpg (PNG→JPEG via PIL)
- resolve_cover() priority: local → MusicBrainz → Discogs
- music_enricher: pass artist/album to resolve_cover() for Discogs lookup

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

cover_handler.py

@@ -221,10 +221,67 @@ def embed_cover(audio_path: Path, cover_path: Path) -> bool:
     return False
 
 
+def _discogs_cover_url(artist: Optional[str], album: Optional[str]) -> Optional[str]:
+    """Sucht auf Discogs nach artist+album und gibt die primäre Image-URL zurück."""
+    if not HAS_REQUESTS or not artist or not album:
+        return None
+    import os
+    token = os.getenv("DISCOGS_TOKEN", "")
+    headers = {"User-Agent": "MusicMetadataEnricher/1.0"}
+    if token:
+        headers["Authorization"] = f"Discogs token={token}"
+    try:
+        r = requests.get(
+            "https://api.discogs.com/database/search",
+            params={"artist": artist, "release_title": album, "type": "release", "per_page": 3},
+            headers=headers,
+            timeout=10,
+        )
+        if r.status_code != 200:
+            return None
+        results = r.json().get("results", [])
+        for result in results:
+            cover = result.get("cover_image") or result.get("thumb")
+            if cover and "spacer" not in cover:
+                return cover
+    except Exception as e:
+        print(f"  ⚠️ Discogs-Suchfehler: {e}", file=sys.stderr)
+    return None
+
+
+def download_discogs_cover(artist: Optional[str], album: Optional[str], dest_dir: Path) -> Optional[Path]:
+    url = _discogs_cover_url(artist, album)
+    if not url:
+        return None
+    dest = dest_dir / "folder.jpg"
+    try:
+        r = requests.get(url, timeout=15, headers={"User-Agent": "MusicMetadataEnricher/1.0"})
+        if r.status_code != 200:
+            return None
+        ct = r.headers.get("content-type", "")
+        if ("png" in ct or url.lower().endswith(".png")) and HAS_PIL:
+            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"  ⚠️ Discogs-Cover-Fehler: {e}", file=sys.stderr)
+        dest.unlink(missing_ok=True)
+    return None
+
+
 def resolve_cover(
     image_files: List[Path],
     release_mbid: Optional[str],
     album_dir: Path,
+    artist: Optional[str] = None,
+    album: Optional[str] = None,
 ) -> tuple[Optional[Path], Optional[str]]:
     """Returns (cover_path, source_label)."""
     local = find_local_cover(image_files)
@@ -237,4 +294,9 @@ def resolve_cover(
         if downloaded:
             return downloaded, "musicbrainz"
 
+    if artist or album:
+        downloaded = download_discogs_cover(artist, album, album_dir)
+        if downloaded:
+            return downloaded, "discogs"
+
     return None, None