Hold TOTP enrollment secret server-side and email user on activation

TOTP enrollment previously round-tripped the generated secret through a
hidden form field and saved whatever the client submitted, letting an
attacker with session access enroll a 2FA device they control by posting
their own secret plus a matching code. Stash the secret in the session
at GET /totp/new, read it only from the session at POST /totp, and drop
the hidden field from the view. Notify the user by email on successful
enrollment so unauthorized activations are visible even if a new vector
appears later.

Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>

app/mailers/totp_mailer.rb

@@ -0,0 +1,7 @@
+class TotpMailer < ApplicationMailer
+  def enabled(user)
+    @user = user
+    mail subject: "Two-factor authentication enabled on your account",
+         to: user.email_address
+  end
+end