Add BoxIMClient with get_credentials and send_p2p_message methods,
using Huihui platform MD5 signing mechanism (sorted keys, 24h timestamp,
12-char alphanumeric nonce). Includes full test coverage.
Add takeover_enabled, takeover_mode, and takeover_delay_seconds columns
to the Authorization model with a SQLite migration, and include them
in to_dict(). Test: test_takeover_model.py.