refactor: extract metadata processing logic to domain layer

- Replace manual metadata extraction with regex in pyqt/conversion.py
  with calls to domain/metadata_extraction.py functions
- Remove duplicate _embed_m4b_metadata and _apply_m4b_chapters_with_mutagen
  functions from webui/conversion_runner.py
- Use ExportService.embed_m4b_metadata for m4b metadata embedding
- Reduce code duplication between PyQt and WebUI interfaces

tests/test_metadata_extraction.py

@@ -0,0 +1,199 @@
+"""Tests for abogen.domain.metadata_extraction module."""
+
+import pytest
+
+from abogen.domain.metadata_extraction import (
+    extract_metadata_from_text,
+    get_filename_from_path,
+    build_ffmpeg_metadata_args,
+    extract_metadata_and_build_args,
+    read_text_for_metadata,
+)
+
+
+class TestExtractMetadataFromText:
+    """Tests for extract_metadata_from_text function."""
+
+    def test_extract_all_metadata(self):
+        """Test extracting all metadata tags."""
+        text = """
+        <<METADATA_TITLE:Test Book>>
+        <<METADATA_ARTIST:Test Author>>
+        <<METADATA_ALBUM:Test Album>>
+        <<METADATA_YEAR:2024>>
+        <<METADATA_ALBUM_ARTIST:Album Artist>>
+        <<METADATA_COMPOSER:Composer Name>>
+        <<METADATA_GENRE:Fiction>>
+        <<METADATA_COVER_PATH:/path/to/cover.jpg>>
+        """
+        metadata = extract_metadata_from_text(text)
+        
+        assert metadata["title"] == "Test Book"
+        assert metadata["artist"] == "Test Author"
+        assert metadata["album"] == "Test Album"
+        assert metadata["year"] == "2024"
+        assert metadata["album_artist"] == "Album Artist"
+        assert metadata["composer"] == "Composer Name"
+        assert metadata["genre"] == "Fiction"
+        assert metadata["cover_path"] == "/path/to/cover.jpg"
+
+    def test_extract_partial_metadata(self):
+        """Test extracting partial metadata."""
+        text = "<<METADATA_TITLE:Only Title>>"
+        metadata = extract_metadata_from_text(text)
+        
+        assert metadata["title"] == "Only Title"
+        assert metadata["artist"] is None
+        assert metadata["cover_path"] is None
+
+    def test_empty_text(self):
+        """Test extracting from empty text."""
+        metadata = extract_metadata_from_text("")
+        
+        for key in metadata:
+            assert metadata[key] is None
+
+    def test_no_tags(self):
+        """Test text without metadata tags."""
+        text = "This is just regular text without any metadata tags."
+        metadata = extract_metadata_from_text(text)
+        
+        for key in metadata:
+            assert metadata[key] is None
+
+    def test_strip_whitespace(self):
+        """Test that values are stripped of whitespace."""
+        text = "<<METADATA_TITLE:  Test Book  >>"
+        metadata = extract_metadata_from_text(text)
+        
+        assert metadata["title"] == "Test Book"
+
+
+class TestGetFilenameFromPath:
+    """Tests for get_filename_from_path function."""
+
+    def test_simple_path(self):
+        """Test extracting filename from simple path."""
+        filename = get_filename_from_path("/path/to/file.txt")
+        assert filename == "file"
+
+    def test_path_with_multiple_extensions(self):
+        """Test extracting filename from path with multiple extensions."""
+        filename = get_filename_from_path("/path/to/file.tar.gz")
+        assert filename == "file.tar"
+
+    def test_windows_path(self):
+        """Test extracting filename from Windows path."""
+        # Note: This test may behave differently on Windows vs Unix
+        # but should work correctly on the current platform
+        filename = get_filename_from_path("C:\\path\\to\\file.txt")
+        assert "file" in filename
+
+    def test_with_display_path(self):
+        """Test using display_path when not from_queue."""
+        filename = get_filename_from_path(
+            file_path="/original/path/file.txt",
+            display_path="/display/path/display_file.txt",
+            from_queue=False,
+        )
+        assert filename == "display_file"
+
+    def test_with_display_path_from_queue(self):
+        """Test ignoring display_path when from_queue."""
+        filename = get_filename_from_path(
+            file_path="/original/path/file.txt",
+            display_path="/display/path/display_file.txt",
+            from_queue=True,
+        )
+        assert filename == "file"
+
+
+class TestBuildFfmpegMetadataArgs:
+    """Tests for build_ffmpeg_metadata_args function."""
+
+    def test_all_metadata_provided(self):
+        """Test building args with all metadata provided."""
+        metadata = {
+            "title": "Test Title",
+            "artist": "Test Artist",
+            "album": "Test Album",
+            "year": "2024",
+            "album_artist": "Album Artist",
+            "composer": "Composer",
+            "genre": "Fiction",
+        }
+        args = build_ffmpeg_metadata_args(metadata, "fallback")
+        
+        assert "-metadata" in args
+        assert "title=Test Title" in args
+        assert "artist=Test Artist" in args
+        assert "album=Test Album" in args
+        assert "date=2024" in args  # year -> date
+
+    def test_use_defaults(self):
+        """Test that defaults are used for missing metadata."""
+        metadata = {}  # Empty metadata
+        args = build_ffmpeg_metadata_args(metadata, "mybook")
+        
+        # Should use defaults
+        assert any("title=mybook" in arg for arg in args)
+        assert any("artist=Unknown" in arg for arg in args)
+        assert any("genre=Audiobook" in arg for arg in args)
+
+    def test_empty_values_skipped(self):
+        """Test that empty values are skipped."""
+        metadata = {
+            "title": "Test",
+            "artist": "",
+            "album": None,
+        }
+        args = build_ffmpeg_metadata_args(metadata, "fallback")
+        
+        # Should have title but not artist/album (empty)
+        assert any("title=Test" in arg for arg in args)
+
+
+class TestExtractMetadataAndBuildArgs:
+    """Tests for extract_metadata_and_build_args function."""
+
+    def test_full_workflow(self):
+        """Test full metadata extraction and arg building."""
+        text = "<<METADATA_TITLE:My Book>>\n<<METADATA_ARTIST:My Author>>"
+        args, cover_path = extract_metadata_and_build_args(
+            text=text,
+            filename="mybook.txt",
+        )
+        
+        assert any("title=My Book" in arg for arg in args)
+        assert any("artist=My Author" in arg for arg in args)
+        assert cover_path is None
+
+    def test_with_cover_path(self):
+        """Test extraction with cover path."""
+        text = "<<METADATA_COVER_PATH:/covers/cover.jpg>>"
+        args, cover_path = extract_metadata_and_build_args(
+            text=text,
+            filename="mybook.txt",
+        )
+        
+        assert cover_path == "/covers/cover.jpg"
+
+
+class TestReadTextForMetadata:
+    """Tests for read_text_for_metadata function."""
+
+    def test_direct_text(self):
+        """Test reading direct text."""
+        text = read_text_for_metadata(
+            file_path="This is direct text",
+            is_direct_text=True,
+        )
+        assert text == "This is direct text"
+
+    def test_file_not_found(self):
+        """Test handling of file not found."""
+        text = read_text_for_metadata(
+            file_path="/nonexistent/file.txt",
+            is_direct_text=False,
+        )
+        assert text == ""