Fix Victron CTR nonce length regression (#316)

[1technophile]

Summary

Fixes #316.

• Reverts VictronDecryptor.compute_nonce() back to padding the nonce to 8 bytes (was changed to 16 in Fully consistent 16 byte long nonce #308).
• Adds a short comment explaining why 8 — and not 16 — is correct, so this regression doesn't get re-introduced under another "consistency" pass.

Why the 16-byte padding broke decryption

pycryptodome's AES.new(..., MODE_CTR, nonce=...) requires the nonce length to be strictly less than the 16-byte AES block size, because the remaining bytes are reserved for the internal counter that CTR mode increments per block. Padding to a full 16 bytes leaves no room for the counter and pycryptodome rejects it with ValueError: Nonce is too long, which is exactly the traceback the issue reporter captured:

File ".../TheengsGateway/decryption.py", line 156, in decrypt
    cipher = AES.new(bindkey, AES.MODE_CTR, nonce=nonce)
ValueError: Nonce is too long

The "consistency with OMG decryption" rationale in #308 does not transfer: the OMG decryptors in this same file use AES.MODE_CCM (lines 70 and 115), which has different nonce-length rules from CTR. CCM and CTR are not API-compatible on this parameter.

Net effect of the regression: every Victron encrypted advertisement has silently failed to decrypt since 1.7.0 (MQTT messages contain "model": "Victron encrypted" with no decoded fields). Confirmed by the reporter on both Pi 5 and Pi 3B against a Victron Blue Smart IP65 12/25 charger; reverting to 8-byte padding restores all expected fields (volt_batt_1, current_batt_1, device_state, error_code).

Test plan

The gateway has no Python test suite to add a regression test to, so verification is manual:

• Build and install this branch on a host with a Victron Blue Smart device in range.
• Confirm decoded fields appear in MQTT (volt_batt_1, device_state, etc.) instead of just "model": "Victron encrypted".
• Confirm no Decryption failed / Nonce is too long errors in the log.

Standing up Python test infrastructure for the decryption module is worth doing in a follow-up so this can't silently regress again, but is out of scope for this one-line revert.

[cnause]

Works as expected with my Victron charger

[1technophile]

@DigiH should we merge this?