Enable ECDH encryption for T8214 and T8425 P2P sessions

[lenoxys]

Enable ECDH encryption for T8214 and T8425 P2P sessions

Summary

T8214 (Battery Doorbell Plus E340) and T8425 (Floodlight Camera) use ECDH key exchange instead of RSA for P2P session encryption. This PR adds ECDH support, gated exclusively to these two devices.

Depends on #793 (device registration + livestream routing).

What was wrong

When the station sends CMD_GATEWAYINFO, T8214/T8425 provide an ECDH public key instead of RSA-encrypted data. The RSA decrypt fails and the session falls back to Level 1 encryption, breaking livestream.

What this PR does

1. Adds ecc_private_key optional field to the Cipher API model.
2. Adds decryptP2PKeyECDH function to derive the session key via ECDH shared secret.
3. In CMD_GATEWAYINFO handling: tries RSA first, falls back to ECDH only if station_sn starts with T8214 or T8425 and ecc_private_key exists.
4. Adds block-alignment guard — skips decryption when payload is not a multiple of 16 bytes (observed with 36-byte status replies from these devices).
5. Unit tests for decryptP2PKeyECDH (6 tests, all passing).

Testing

• ECDH key exchange tested end-to-end on T8214 and T8425.
• 6 unit tests for decryptP2PKeyECDH covering compressed/uncompressed pubkeys, ECIES envelopes, determinism, and output size.

[max246]

Might hold off on this one until we release tomorrow, so many things merged and I am running the develop for now to ensure it works