[IMP] storage_backend_sftp: security, compatibility and debugging

[IJOL]

Summary

• Add host key verification to prevent MITM attacks
• Flexible key input accepting file paths, PEM content, or bytes
• Legacy SSH algorithm support for older SFTP servers (banking, etc.)
• Verbose logging for connection troubleshooting
• Atomic file get operations using tempfile (prevents partial reads)

New fields: sftp_verify_hostkey, sftp_hostkey, sftp_legacy_algorithms, sftp_verbose_logging

Test plan

• Install storage_backend_sftp on a clean database
• Configure an SFTP backend with password authentication — verify connection works
• Configure with SSH key (file path and direct content) — verify both work
• Enable verbose logging — verify debug output in logs
• Enable host key verification with correct key — verify connection succeeds
• Enable host key verification with wrong key — verify connection is rejected
• Enable legacy algorithms — verify connection to older servers works
• Test file get operation — verify atomic download via tempfile