refactor(domain): deduplicate metadata helpers across 3 layers

Extracted 8 metadata functions from service.py, exporters.py, and
audiobookshelf.py into domain/metadata_helpers.py:
- normalize_metadata_casefold, split_people_field, split_simple_list
- first_nonempty, extract_year, normalize_series_sequence
- build_audiobookshelf_metadata, load_audiobookshelf_chapters

service.py, exporters.py, and audiobookshelf.py now import from domain
instead of maintaining separate copies. Thin wrappers adapt to layer
interfaces (Job objects, etc.).

Net -231 lines. 1006 tests pass.

tests/test_metadata_helpers_abs.py

@@ -0,0 +1,170 @@
+"""Tests for domain/metadata_helpers.py Audiobookshelf helpers."""
+from abogen.domain.metadata_helpers import (
+    normalize_metadata_casefold,
+    split_people_field,
+    split_simple_list,
+    first_nonempty,
+    extract_year,
+    normalize_series_sequence,
+    build_audiobookshelf_metadata,
+    load_audiobookshelf_chapters,
+)
+
+
+# --- normalize_metadata_casefold ---
+
+def test_normalize_metadata_casefold_basic():
+    result = normalize_metadata_casefold({"Title": "  Hello  ", "Author": None, "": "skip"})
+    assert result == {"title": "Hello", "author": ""} or result == {"title": "Hello"}
+
+
+def test_normalize_metadata_casefold_preserves_lists():
+    result = normalize_metadata_casefold({"tags": ["a", "b"]})
+    assert result["tags"] == ["a", "b"]
+
+
+# --- split_people_field ---
+
+def test_split_people_field_single():
+    assert split_people_field("J.K. Rowling") == ["J.K. Rowling"]
+
+
+def test_split_people_field_multiple():
+    result = split_people_field("Tolkien, Lewis & Martin")
+    assert "Tolkien" in result
+    assert "Lewis" in result
+    assert "Martin" in result
+
+
+def test_split_people_field_deduplicates():
+    result = split_people_field("Tolkien, tolkien, TOLKIEN")
+    assert len(result) == 1
+
+
+def test_split_people_field_none():
+    assert split_people_field(None) == []
+
+
+def test_split_people_field_list():
+    result = split_people_field(["Author A", "Author B"])
+    assert result == ["Author A", "Author B"]
+
+
+# --- split_simple_list ---
+
+def test_split_simple_list_basic():
+    result = split_simple_list("fantasy, sci-fi; thriller")
+    assert "fantasy" in result
+    assert "sci-fi" in result
+    assert "thriller" in result
+
+
+def test_split_simple_list_none():
+    assert split_simple_list(None) == []
+
+
+# --- first_nonempty ---
+
+def test_first_nonempty_basic():
+    assert first_nonempty(None, "", "hello") == "hello"
+
+
+def test_first_nonempty_first_wins():
+    assert first_nonempty("first", "second") == "first"
+
+
+def test_first_nonempty_none():
+    assert first_nonempty(None, None) is None
+
+
+def test_first_nonempty_list():
+    assert first_nonempty(None, ["a", "b"]) == "a"
+
+
+# --- extract_year ---
+
+def test_extract_year_full_date():
+    assert extract_year("Published on 2023-05-15") == 2023
+
+
+def test_extract_year_plain():
+    assert extract_year("2020") == 2020
+
+
+def test_extract_year_none():
+    assert extract_year(None) is None
+
+
+def test_extract_year_invalid():
+    assert extract_year("no year here") is None
+
+
+# --- normalize_series_sequence ---
+
+def test_normalize_series_sequence_int():
+    assert normalize_series_sequence(3) == "3"
+
+
+def test_normalize_series_sequence_float():
+    assert normalize_series_sequence(2.5) == "2.5"
+
+
+def test_normalize_series_sequence_string():
+    assert normalize_series_sequence("  12  ") == "12"
+
+
+def test_normalize_series_sequence_comma():
+    assert normalize_series_sequence("1,5") == "1.5"
+
+
+def test_normalize_series_sequence_none():
+    assert normalize_series_sequence(None) is None
+
+
+def test_normalize_series_sequence_nan():
+    import math
+    assert normalize_series_sequence(float("nan")) is None
+
+
+# --- build_audiobookshelf_metadata ---
+
+def test_build_audiobookshelf_metadata_basic():
+    tags = {
+        "title": "My Book",
+        "author": "Author Name",
+        "description": "A great book",
+    }
+    result = build_audiobookshelf_metadata(tags, language="en")
+    assert result["title"] == "My Book"
+    assert result["authors"] == ["Author Name"]
+    assert result["language"] == "en"
+
+
+def test_build_audiobookshelf_metadata_series():
+    tags = {
+        "title": "Book 2",
+        "series": "My Series",
+        "series_index": "2",
+    }
+    result = build_audiobookshelf_metadata(tags, language="en")
+    assert result["seriesName"] == "My Series"
+    assert result["seriesSequence"] == "2"
+
+
+def test_build_audiobookshelf_metadata_fallback_title():
+    tags = {"author": "Someone"}
+    result = build_audiobookshelf_metadata(tags, language="en", filename="chapter1")
+    assert result["title"] == "chapter1"
+
+
+def test_build_audiobookshelf_metadata_empty():
+    result = build_audiobookshelf_metadata({}, language="en")
+    assert result["language"] == "en"
+    assert "authors" not in result  # empty list stripped
+
+
+def test_build_audiobookshelf_metadata_strips_empty():
+    tags = {"title": "Book", "subtitle": "", "description": None}
+    result = build_audiobookshelf_metadata(tags, language="en")
+    assert "subtitle" not in result
+    assert "description" not in result