fix: review findings on the login throttle

- The throttle runs on a 64-bit clock in ms (esp_timer_get_time()), so a
  slot left alone for 49.7 days isn't brought back by millis() wrapping
  round to its old failure.
- Web.cpp turns the client into the throttle's key in one place, and
  says that an IPv6 client would share key 0; names are clearer
  (retryMs, vacantSlot).
- The shared slot's test now succeeds from the IP counted in it, so a
  success that cleared the shared slot would fail it.
- GLOSSARY.md names it the login throttle, with lockouts, and
  SECURITY_PLAN.md and ADR 0006 use that name.

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

GLOSSARY.md

@@ -148,6 +148,10 @@ _Avoid_: Secure Config Only (removed)
 What logging in returns. It stands for the PIN or password, on one client address, and every request under Secured carries it.
 _Avoid_: session, API key (the name of the field it travels in)
 
+**Login throttle**
+How the device slows down guessing: a client address with too many failed logins is refused for a while, and each refusal is a *lockout*, longer than the one before.
+_Avoid_: login backoff, rate limit
+
 ## Domain modules
 
 **Domain module**