refactor: extract book metadata logic from PyQt to domain

- domain/metadata_extraction.py: add format_metadata_tags(),
  extract_book_metadata_epub(), extract_book_metadata_pdf(),
  extract_book_metadata_markdown(), _save_cover_to_cache()
- pyqt/book_handler.py: _extract_book_metadata() reduced from ~165 lines
  to ~10 lines by delegating to domain; _format_metadata_tags() reduced
  from ~55 lines to ~15 lines; ebooklib/fitz imports moved to domain
- +16 tests (format_metadata_tags, save_cover, markdown extraction)
- 1169 tests pass

tests/test_domain_metadata_book.py

@@ -0,0 +1,114 @@
+"""Tests for domain/metadata_extraction.py — format_metadata_tags, extract_book_metadata_*."""
+
+import os
+
+import pytest
+
+from abogen.domain.metadata_extraction import (
+    extract_book_metadata_markdown,
+    format_metadata_tags,
+    _save_cover_to_cache,
+)
+
+
+class TestFormatMetadataTags:
+    def test_basic_epub(self):
+        metadata = {
+            "title": "My Book",
+            "authors": ["Author One", "Author Two"],
+            "publication_year": "2023",
+        }
+        result = format_metadata_tags(metadata, "fallback", 10, "epub")
+        assert "<<METADATA_TITLE:My Book>>" in result
+        assert "<<METADATA_ARTIST:Author One, Author Two>>" in result
+        assert "<<METADATA_ALBUM:My Book (10 Chapters)>>" in result
+        assert "<<METADATA_YEAR:2023>>" in result
+        assert "<<METADATA_GENRE:Audiobook>>" in result
+
+    def test_pdf_uses_pages(self):
+        metadata = {"title": "PDF Doc", "authors": ["Writer"]}
+        result = format_metadata_tags(metadata, "doc", 50, "pdf")
+        assert "50 Pages" in result
+
+    def test_markdown_uses_chapters(self):
+        metadata = {"title": "MD Doc"}
+        result = format_metadata_tags(metadata, "doc", 3, "markdown")
+        assert "3 Chapters" in result
+
+    def test_fallback_title(self):
+        metadata = {}
+        result = format_metadata_tags(metadata, "fallback_name", 1, "epub")
+        assert "<<METADATA_TITLE:fallback_name>>" in result
+
+    def test_unknown_authors(self):
+        metadata = {"authors": []}
+        result = format_metadata_tags(metadata, "file", 1, "epub")
+        assert "<<METADATA_ARTIST:Unknown>>" in result
+
+    def test_cover_bytes_saved(self, tmp_path):
+        metadata = {"title": "With Cover"}
+        cover = b"\x89PNG\r\n\x1a\n" + b"\x00" * 100  # Fake image bytes
+        result = format_metadata_tags(
+            metadata, "file", 1, "epub",
+            cover_bytes=cover, cache_dir=str(tmp_path),
+        )
+        assert "<<METADATA_COVER_PATH:" in result
+        # Verify file was created
+        cover_files = list(tmp_path.glob("cover_*.jpg"))
+        assert len(cover_files) == 1
+        assert cover_files[0].read_bytes() == cover
+
+    def test_no_cover_bytes(self):
+        metadata = {"title": "No Cover"}
+        result = format_metadata_tags(metadata, "file", 1, "epub")
+        assert "METADATA_COVER_PATH" not in result
+
+    def test_authors_as_string(self):
+        metadata = {"authors": "Single Author"}
+        result = format_metadata_tags(metadata, "file", 1, "epub")
+        assert "<<METADATA_ARTIST:Single Author>>" in result
+
+
+class TestSaveCoverToCache:
+    def test_saves_file(self, tmp_path):
+        data = b"\x89PNG" + b"\x00" * 50
+        result = _save_cover_to_cache(data, str(tmp_path))
+        assert result is not None
+        assert os.path.exists(result)
+        assert open(result, "rb").read() == data
+
+    def test_none_bytes(self, tmp_path):
+        assert _save_cover_to_cache(None, str(tmp_path)) is None
+
+    def test_none_cache_dir(self):
+        assert _save_cover_to_cache(b"data", None) is None
+
+    def test_returns_normalized_path(self, tmp_path):
+        result = _save_cover_to_cache(b"data", str(tmp_path))
+        assert result == os.path.normpath(result)
+
+
+class TestExtractBookMetadataMarkdown:
+    def test_frontmatter(self):
+        text = "---\ntitle: Test Title\nauthor: Test Author\ndate: 2024\n---\n\nBody"
+        result = extract_book_metadata_markdown(text)
+        assert result["title"] == "Test Title"
+        assert result["authors"] == ["Test Author"]
+        assert result["publication_year"] == "2024"
+
+    def test_fallback_to_h1(self, ):
+        text = "# My Heading\n\nSome content"
+        toc = [{"level": 1, "name": "My Heading"}]
+        result = extract_book_metadata_markdown(text, toc)
+        assert result["title"] == "My Heading"
+
+    def test_empty_text(self):
+        result = extract_book_metadata_markdown("")
+        assert result["title"] is None
+        assert result["authors"] == []
+
+    def test_frontmatter_with_quotes(self):
+        text = '---\ntitle: "Quoted Title"\nauthor: \'Quoted Author\'\n---\n\nBody'
+        result = extract_book_metadata_markdown(text)
+        assert result["title"] == "Quoted Title"
+        assert result["authors"] == ["Quoted Author"]