fix SSH MITM, file permissions, pin dependencies

[kpshukla]

Summary

• Paramiko SSH MITM fix — replaced AutoAddPolicy() with load_system_host_keys() + RejectPolicy() in gui/views.py. The SSH client now loads the user's ~/.ssh/known_hosts and rejects any unrecognised host, preventing man-in-the-middle attacks on LAN SSH connections.
• Password file permissions — data/lndg-admin.txt is now created with mode=0o600 and os.chmod applied after write, restricting access to owner only (was world-readable by default).
• Pin dependencies — added minimum version bounds to all packages in requirements.txt (e.g. Django>=4.2,<5.0, cryptography>=41.0, paramiko>=3.4) to prevent installation of vulnerable or breaking versions.

Notes

These fixes are scoped specifically to the local/single-user LNDg deployment model (localhost, LAN only). Other potential findings (open redirect, query param handling, security headers) were reviewed and deemed low/negligible risk given that context.

Test plan

• Verify SSH channel DB size feature still works for hosts present in ~/.ssh/known_hosts
• Run pip install -r requirements.txt to confirm no dependency conflicts
• Run a fresh initialize.py and confirm data/lndg-admin.txt has permissions 600

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