fix(auth)!: change timestamp to ms since Unix epoch for effective replay protection

[VasilevNStas]

Description

Changes Auth.timestamp from uint32 (minutes since project epoch) to uint64 timestamp_ms (milliseconds since Unix epoch) for effective per-message replay protection.

Problem

The previous uint32 timestamp used minute granularity. Every Auth sent within the same 60-second window carried the identical value. With the ±1 minute acceptance window, a captured frame could be replayed for up to 119 seconds. This is not a nonce — it's a coarse time window.

Fix

Before	After
uint32 timestamp = 2	uint64 timestamp_ms = 2
Minutes since 2024-01-01 UTC	Milliseconds since Unix epoch
±1 minute acceptance	~1–2 second skew window
HMAC input: 4 bytes LE/BE undefined	HMAC input: 8 bytes big-endian

The big-endian serialization is now explicitly specified in the HMAC comment.

Migration

Breaking wire change — the Auth frame format changes. All Nodes and clients must update simultaneously:

• Auth field 2 wire type changes (varint fits within 1–10 bytes)
• HMAC input length changes from 4 to 8 bytes
• Generated code symbol changes from timestamp to timestamp_ms

Closes #29

[VasilevNStas]

@yegor256 @davvd plz review this PR

[VasilevNStas]

@davvd conflicts resolved.