feat(05-04): implement WebDAVBackend with SSRF guard and asyncio wrapping
- All 7 StorageBackend methods implemented as async coroutines
- validate_cloud_url() called in __init__ (SSRF at construct time) and before
every asyncio.to_thread() call (D-17 defense-in-depth / T-05-04-01, T-05-04-02)
- _make_path() builds "docuvault/{user_id}/{document_id}{ext}" with urllib.parse.quote
encoding on path segments (RESEARCH.md Pitfall 2)
- presigned_get_url and generate_presigned_put_url raise NotImplementedError (D-14)
- All webdavclient3 sync calls (upload_to, download_from, clean, info, check, mkdir)
wrapped in asyncio.to_thread() per MinIOBackend pattern
- delete_object silently ignores missing file exceptions (StorageBackend ABC contract)
curo1305
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