feat: Enhanced anime add flow with sanitized folders and targeted scan

- Add sanitize_folder_name utility for filesystem-safe folder names
- Add sanitized_folder property to Serie entity
- Update SerieList.add() to use sanitized display names for folders
- Add scan_single_series() method for targeted episode scanning
- Enhance add_series endpoint: DB save -> folder create -> targeted scan
- Update response to include missing_episodes and total_missing
- Add comprehensive unit tests for new functionality
- Update API tests with proper mock support

tests/unit/test_serie_class.py

@@ -320,3 +320,96 @@ class TestSerieDeprecationWarnings:
         finally:
             if os.path.exists(temp_filename):
                 os.remove(temp_filename)
+
+
+class TestSerieSanitizedFolder:
+    """Test Serie.sanitized_folder property."""
+
+    def test_sanitized_folder_from_name(self):
+        """Test that sanitized_folder uses the name property."""
+        serie = Serie(
+            key="attack-on-titan",
+            name="Attack on Titan: Final Season",
+            site="aniworld.to",
+            folder="old-folder",
+            episodeDict={}
+        )
+        
+        result = serie.sanitized_folder
+        assert ":" not in result
+        assert "Attack on Titan" in result
+
+    def test_sanitized_folder_removes_special_chars(self):
+        """Test that special characters are removed."""
+        serie = Serie(
+            key="re-zero",
+            name="Re:Zero - Starting Life in Another World?",
+            site="aniworld.to",
+            folder="old-folder",
+            episodeDict={}
+        )
+        
+        result = serie.sanitized_folder
+        assert ":" not in result
+        assert "?" not in result
+
+    def test_sanitized_folder_fallback_to_folder(self):
+        """Test fallback to folder when name is empty."""
+        serie = Serie(
+            key="test-key",
+            name="",
+            site="aniworld.to",
+            folder="Valid Folder Name",
+            episodeDict={}
+        )
+        
+        result = serie.sanitized_folder
+        assert result == "Valid Folder Name"
+
+    def test_sanitized_folder_fallback_to_key(self):
+        """Test fallback to key when name and folder can't be sanitized."""
+        serie = Serie(
+            key="valid-key",
+            name="",
+            site="aniworld.to",
+            folder="",
+            episodeDict={}
+        )
+        
+        result = serie.sanitized_folder
+        assert result == "valid-key"
+
+    def test_sanitized_folder_preserves_unicode(self):
+        """Test that Unicode characters are preserved."""
+        serie = Serie(
+            key="japanese-anime",
+            name="進撃の巨人",
+            site="aniworld.to",
+            folder="old-folder",
+            episodeDict={}
+        )
+        
+        result = serie.sanitized_folder
+        assert "進撃の巨人" in result
+
+    def test_sanitized_folder_with_various_anime_titles(self):
+        """Test sanitized_folder with real anime titles."""
+        test_cases = [
+            ("fate-stay-night", "Fate/Stay Night: UBW"),
+            ("86-eighty-six", "86: Eighty-Six"),
+            ("steins-gate", "Steins;Gate"),
+        ]
+        
+        for key, name in test_cases:
+            serie = Serie(
+                key=key,
+                name=name,
+                site="aniworld.to",
+                folder="old-folder",
+                episodeDict={}
+            )
+            result = serie.sanitized_folder
+            # Verify invalid filesystem characters are removed
+            # Note: semicolon is valid on Linux but we test common invalid chars
+            assert ":" not in result
+            assert "/" not in result