fix(email): explicit TLS mode for non-standard relay ports

The 465-only implicit-TLS heuristic mis-classified Selectel's SSL port (1127),
which the mailer would have dialled with STARTTLS and failed. Add BACKEND_SMTP_TLS
(ssl|starttls) — empty still derives the mode from the port (implicit on 465, else
STARTTLS) — and dial implicit TLS with WithSSL()+WithPort so any port works, not
just 465. Wire SMTP_RELAY_TLS through compose/ci/prod/.env.example and document it
(Selectel: 1127 = SSL, 1126 = STARTTLS). Unit-tested.

backend/internal/config/config.go

@@ -148,6 +148,7 @@ func Load() (Config, error) {
 		Username: os.Getenv("BACKEND_SMTP_USERNAME"),
 		Password: os.Getenv("BACKEND_SMTP_PASSWORD"),
 		From:     envOr("BACKEND_SMTP_FROM", "no-reply@localhost"),
+		TLS:      os.Getenv("BACKEND_SMTP_TLS"),
 	}
 
 	c := Config{