Improve DDG fallback for Alpina book metadata

[gman-dev-nov]

Summary

• teach the DuckDuckGo ISBN fallback to extract authors from result snippets when titles do not include them
• clean noisy marketplace-style titles for Alpina/ANF cases and prefer cleaner candidates when DDG returns multiple variants
• add focused provider tests for the specific ISBN 9785916717099 plus the new title-cleaning rules

Testing

• venv/bin/python -m pytest tests/test_home_library_providers.py -q
• venv/bin/python -m py_compile home_library/providers/ddg.py tests/test_home_library_providers.py