relay: fix timing attack on password authentication (GHSA-vhv8-g2r9-cwcc)

The relay authentication used non-constant-time comparisons (strcasecmp,
strcmp) to verify password hashes and plaintext passwords, allowing an
attacker to derive the expected hash byte-by-byte from response timing
and then authenticate without knowing the password.

- SHA/PBKDF2 hex hash comparisons: normalize the client-supplied hash to
  uppercase and compare in constant time over the fixed expected length.
- Plaintext password comparison: HMAC-SHA256 both passwords with a fresh
  per-call random key and compare the fixed-size MACs in constant time,
  hiding both per-byte timing and the password length.

Add string_memcmp_constant_time helper in core, exposed via the plugin
API. Bump WEECHAT_PLUGIN_API_VERSION accordingly.

CHANGELOG.md

@@ -23,6 +23,7 @@ SPDX-License-Identifier: GPL-3.0-or-later
 - irc: fix tag in message with list of names when joining a channel
 - fset: remove error displayed in core buffer when clicking with the mouse below the last option displayed
 - relay: limit size of decompressed websocket frame with permessage-deflate to prevent memory exhaustion ([GHSA-v2v4-45wm-5cr3](https://github.com/weechat/weechat/security/advisories/GHSA-v2v4-45wm-5cr3))
+- relay: fix timing attack on password authentication ([GHSA-vhv8-g2r9-cwcc](https://github.com/weechat/weechat/security/advisories/GHSA-vhv8-g2r9-cwcc))
 
 ## Version 4.9.0 (2026-03-29)