Bind remote reads to opened SFTP and SMB objects

[bifrost0x]

Summary

• introduce a protocol-neutral reader lease that keeps remote metadata bound to the exact SFTP or SMB handle supplying bytes
• use SFTP handle stat and SMB2 CREATE metadata, reject invalid or non-readable opened objects, and preserve separate transfer I/O lanes
• open downloads before response construction, disable byte ranges, enforce declared and observed byte limits, and close handles on every response outcome
• apply the same binding to previews, editor reads, archive members, remote ZIP streams, and server-to-server copies

Security boundaries

• WebSSH does not add a virtual allowed-folder root for SFTP; deployments that require subtree confinement must use the remote account and server-side chroot
• supported SMB targets must continue to disable symlink and wide-link traversal; final read handles are also opened without following reparse points and validated from CREATE metadata
• RBAC and role-based connection visibility are intentionally not part of this change

Validation

• pytest tests/test_binary_transfer_preview.py tests/test_file_backend.py tests/test_remote_transfer.py tests/test_sftp_backend.py tests/test_sftp_handler.py tests/test_smb_backend.py tests/test_transfer_cancellation.py tests/test_transfer_routes.py -q — 221 passed
• pytest tests -q — 2,213 passed, 33 skipped
• python scripts/run_integration_tests.py — 27 passed against disposable OpenSSH targets
• python scripts/run_smb_integration_tests.py — SMB 3.1.1 integration checks passed
• security diff review — 0 findings across 7 changed production modules