auth: reset links pause for a day after five (audit A3)

Security audit 2026-10-17, finding A3 (low), stage 3. sendReset spaced
reset mails ten minutes apart and capped them at five an hour per account,
so a stranger who repeated a username kept a live 30-minute link arriving
in the administrator's mailbox five times an hour, for ever.

Now five reset links a day per account (resetDayLimiter); the sixth
request sends one notice instead - "Reset links paused for a day", no
link, saying further requests are ignored for 24 hours and that a global
administrator can set a password on Server > Users - and every later one
in the window sends nothing (resetNoticeLimiter), logged either way. In
memory, like every limiter: a restart clears them.

Test: TestResetLinksPauseForADayAfterFive. Docs: security.md, guide §
Account; CHANGELOG; plan.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -22,6 +22,10 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
   and name, and an IPv6 client is one per /64. With every request coming
   from one private address with `X-Forwarded-For` and `TRUSTED_PROXY_CIDR`
   empty, the log says what to set. Refused sign-ins are logged.
+- **Reset links pause for a day after five** (audit A3): anyone who knew a
+  username could keep a live reset link arriving in that mailbox five times
+  an hour. After five in a day the owner is told once that further requests
+  are ignored for 24 hours.
 - **The TLS private key no longer has to be readable by Postfix** (audit
   I1): the guide gave it to group 10998 for the panel's backup, and
   `postfix` is in that group, so every Postfix daemon - an smtpd talking to