New files:
- filevault.py: Hash-based file storage with use_pairs option (v1.1.0)
- async_filevault.py: Async wrapper using asyncio.to_thread()
- domain_vault.py: Triple vault system for web archival (HTML, Media, Linkpeek)
- screenshot.py: Async screenshot capture using uri2png
- tests/unit/: Comprehensive test suite (85 tests)

Sync-to-async conversions:
- storage.py: Wrap Path operations in asyncio.to_thread()
- domain_vault.py: Wrap exists(), mkdir(), rglob(), os.walk() in asyncio.to_thread()
- screenshot.py: Wrap read_bytes(), write_bytes(), unlink() in asyncio.to_thread()

All sync filesystem operations now run in thread pool to avoid blocking async loop.