Add 4 new cover/tracklist sources: MB back cover, iTunes, Last.fm, Discogs tracklist

cover_handler.py:
- _download_image(): shared helper replaces duplicated download logic
- download_back_cover(): fetches back cover from MusicBrainz CAA (/back endpoint),
  saves as back.jpg; skips if already present
- _itunes_cover_url() / download_itunes_cover(): iTunes Search API (no auth),
  requests 600x600 artwork; fallback after Discogs
- _lastfm_cover_url() / download_lastfm_cover(): Last.fm album.getinfo
  (LASTFM_API_KEY env var); last cover fallback, skips placeholder images
- resolve_cover(): extended with iTunes → Last.fm fallback chain

metadata_resolver.py:
- _discogs_get_tracklist(): fetches full Discogs release via REST API,
  parses tracklist[] including heading-based disc detection
- _lastfm_tracklist(): fetches Last.fm album.getinfo tracks (LASTFM_API_KEY)
- resolve(): uses Discogs tracklist → Last.fm tracklist as fallback when
  MusicBrainz returns no tracks; LASTFM_API_KEY added to env var block

music_enricher.py:
- process_album(): calls download_back_cover() after execute_album() when MBID known

New cover priority:  local → MusicBrainz front → Discogs → iTunes → Last.fm
New tracklist priority: local → YouTube → MusicBrainz → Discogs → Last.fm → OCR
Test suite: 29 → 33 tests (all pass)

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

test_suite_enricher.py

@@ -322,6 +322,74 @@ def test_normalize_cover_renames_front_jpg() -> str:
     return "Front.jpg → folder.jpg rename OK"
 
 
+# ---------------------------------------------------------------------------
+# New cover sources Tests
+# ---------------------------------------------------------------------------
+
+def test_itunes_cover_url_format() -> str:
+    from cover_handler import _itunes_cover_url
+    # Ohne echten Netzwerkaufruf: testen ob Funktion bei leeren Eingaben None zurückgibt
+    assert _itunes_cover_url(None, None) is None, "None inputs → None"
+    assert _itunes_cover_url("", "") is None, "empty inputs → None"
+    return "iTunes URL helper: None-Handling OK"
+
+
+def test_discogs_tracklist_format() -> str:
+    from metadata_resolver import _discogs_get_tracklist
+    # Simuliere API-Antwort-Parsing mit einem Testfall
+    import unittest.mock as mock
+    fake_response = {
+        "tracklist": [
+            {"position": "1", "type_": "track", "title": "Song A", "duration": "3:20"},
+            {"type_": "heading", "title": "CD 2"},
+            {"position": "1", "type_": "track", "title": "Song B", "duration": "4:00"},
+        ]
+    }
+    with mock.patch("requests.get") as mock_get:
+        mock_get.return_value.status_code = 200
+        mock_get.return_value.json.return_value = fake_response
+        tracks = _discogs_get_tracklist(12345)
+    assert len(tracks) == 2, f"expected 2 tracks, got {len(tracks)}"
+    assert tracks[0]["title"] == "Song A", f"track 0: {tracks[0]}"
+    assert tracks[1]["disc"] == 2, f"disc should be 2 after heading: {tracks[1]}"
+    return f"Discogs tracklist format OK: {len(tracks)} tracks"
+
+
+def test_lastfm_tracklist_format() -> str:
+    from metadata_resolver import _lastfm_tracklist
+    import unittest.mock as mock, os
+    fake_response = {
+        "album": {
+            "tracks": {
+                "track": [
+                    {"name": "Track One", "@attr": {"rank": "1"}, "artist": {"name": "Artist"}},
+                    {"name": "Track Two", "@attr": {"rank": "2"}, "artist": {"name": "Artist"}},
+                ]
+            }
+        }
+    }
+    with mock.patch.dict(os.environ, {"LASTFM_API_KEY": "testkey"}):
+        with mock.patch("requests.get") as mock_get:
+            mock_get.return_value.status_code = 200
+            mock_get.return_value.json.return_value = fake_response
+            tracks = _lastfm_tracklist("Artist", "Album")
+    assert len(tracks) == 2, f"expected 2 tracks, got {len(tracks)}"
+    assert tracks[0]["title"] == "Track One", f"track 0: {tracks[0]}"
+    assert tracks[0]["number"] == 1, f"rank/number: {tracks[0]}"
+    return f"Last.fm tracklist format OK: {len(tracks)} tracks"
+
+
+def test_back_cover_skips_if_exists() -> str:
+    from cover_handler import download_back_cover
+    with tempfile.TemporaryDirectory() as tmpdir:
+        root = Path(tmpdir)
+        back = root / "back.jpg"
+        back.write_bytes(b"\xff\xd8" + b"\x00" * 200)
+        result = download_back_cover("fake-mbid", root)
+        assert result == back, f"should return existing back.jpg: {result}"
+    return "back cover skip-if-exists OK"
+
+
 # ---------------------------------------------------------------------------
 # executor Tests
 # ---------------------------------------------------------------------------
@@ -399,6 +467,10 @@ def main() -> None:
         ("UNIT_27_is_classical_false_pop",            test_is_classical_false_for_pop),
         ("UNIT_28_is_classical_false_folk",           test_is_classical_false_for_folk),
         ("UNIT_29_normalize_cover_renames",           test_normalize_cover_renames_front_jpg),
+        ("UNIT_30_itunes_url_none_handling",          test_itunes_cover_url_format),
+        ("UNIT_31_discogs_tracklist_format",          test_discogs_tracklist_format),
+        ("UNIT_32_lastfm_tracklist_format",           test_lastfm_tracklist_format),
+        ("UNIT_33_back_cover_skip_if_exists",         test_back_cover_skips_if_exists),
     ]
 
     for test_id, fn in cases: