test+docs(email): squat-fix coverage and the email-pipeline docs

Add an integration test asserting the guest-until-confirmed squat fix (an
email-login account is a reapable guest until the code is confirmed, then
durable). Document the branded relay pipeline in ARCHITECTURE (go-mail, TLS by
port, no client cert, PUBLIC_BASE_URL anti-injection, per-recipient send throttle,
guest-until-confirmed), FUNCTIONAL (+ru), TESTING and the backend README config
table (BACKEND_SMTP_* + BACKEND_PUBLIC_BASE_URL).

docs/TESTING.md

@@ -69,7 +69,10 @@ tests or touching CI.
   content and block-visibility rules, the nudge turn/rate-limit rules, the
   invitation flow (all-accept starts the game, decline cancels, lazy expiry,
   inviter-only cancel), and the email confirm-code flow (request/confirm, taken
-  email, expiry and attempt-cap) with a fixture mailer. It also covers the
+  email, expiry and attempt-cap, and the guest-until-confirmed login) with a fixture
+  mailer. The branded email template render (en/ru subject, code and body) and the
+  per-recipient send-rate limiter (cooldown + hourly cap) are pure account-package
+  unit tests, needing no database. It also covers the
   **befriend-an-opponent** gate (a request needs a shared game), the **permanent
   decline** and 30-day re-send rule, the **one-time friend code** (issue/redeem,
   self/single-use, decline-bypass), `ListInvitations`, the zero-value `GetStats`, and