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/FUNCTIONAL.md

@@ -61,6 +61,12 @@ A **VK Mini App** launch works the same way: it authenticates from VK's signed l
 the name in the signed launch). While the theme preference is "auto" the app follows the VK
 client's light/dark scheme, and the layout clears the VK mobile home bar. The same quiet-retry
 "couldn't load" screen applies inside VK.
+**Email** sign-in (web) mails a branded six-digit confirmation code — localized
+(en/ru), no images — to the address; entering it signs the player in. A new address
+is provisioned on the first request but only becomes a durable account once the code
+is confirmed, so an abandoned, never-confirmed attempt is cleaned up and its address
+freed. Sends are rate-limited (a short cooldown between codes and a small hourly cap
+per address), so a mistyped address or an impatient tap cannot flood an inbox.
 Telegram runs a **single bot**: every player uses
 the same bot, and all of its chat and out-of-app notifications are written in the
 player's own **interface language** (en/ru). A separate optional **promo bot** can run alongside the