Add SMS consent logging for Twilio compliance

Log phone, IP, and user agent to sms_consents table before sending
verification SMS. Add disclosure text to login form.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

auth.go

@@ -121,6 +121,11 @@ func (s *Server) handleLoginSubmit(w http.ResponseWriter, r *http.Request) {
 			log.Printf("failed to send verification email to %s: %v", identifier, err)
 		}
 	} else {
+		s.q.CreateSmsConsent(r.Context(), db.CreateSmsConsentParams{
+			Phone:     identifier,
+			IpAddress: r.RemoteAddr,
+			UserAgent: r.UserAgent(),
+		})
 		if err := sendVerificationSMS(identifier, code); err != nil {
 			log.Printf("failed to send verification SMS to %s: %v", identifier, err)
 		}