Fix and expand tests: 63 tests passing, covers all core modules

Remove tests/ from .gitignore (was accidentally excluded).

- test_ripper.py: rewrite for current API (_parse_cddb_lines,
  _extract_tracks, _rename_files, _clean_input); fix default quality
- test_organizer.py: update filename assertions (spaces→underscores);
  add TestSanitizeFilename, TestCheckDiscCounts, in-place mode
- test_playlist.py: fix dummy filenames to underscore scheme;
  add multi-disc, filename sanitization, EXTINF and fallback tests

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tests/test_models.py

@@ -0,0 +1,42 @@
+"""Tests für die Datenmodelle."""
+
+from musiksammlung.models import Album
+
+
+def test_album_folder_name_with_year():
+    album = Album(artist="Test", album="Mein Album", year=1987, discs=[])
+    assert album.folder_name == "Mein Album (1987)"
+
+
+def test_album_folder_name_without_year():
+    album = Album(artist="Test", album="Mein Album", year=None, discs=[])
+    assert album.folder_name == "Mein Album"
+
+
+def test_sanitize_name():
+    album = Album(artist='Art:ist', album='Al/bum?', year=None, discs=[])
+    assert ":" not in album.artist
+    assert "/" not in album.album
+    assert "?" not in album.album
+
+
+def test_album_from_json():
+    data = {
+        "artist": "Die Toten Hosen",
+        "album": "Opium fürs Volk",
+        "year": 1996,
+        "discs": [
+            {
+                "disc_number": 1,
+                "tracks": [
+                    {"track_number": 1, "title": "Bonnie & Clyde"},
+                    {"track_number": 2, "title": "Zehn kleine Jägermeister"},
+                ],
+            }
+        ],
+    }
+    album = Album.model_validate(data)
+    assert album.artist == "Die Toten Hosen"
+    assert len(album.discs) == 1
+    assert len(album.discs[0].tracks) == 2
+    assert album.discs[0].tracks[1].title == "Zehn kleine Jägermeister"