Notify users out-of-band when security settings change

Previously only TOTP-enabled triggered an email. Every other
security-relevant change — password change, TOTP disable, passkey
add/remove, API key create/revoke, email address change, backup-code
regeneration — happened silently, so an attacker on a stolen session
could quietly drop 2FA or hijack the email with no signal to the
account holder.

Add SecurityMailer with one method per event. Each email carries the
request IP, user-agent, and timestamp so the user can spot unfamiliar
activity. Email-address changes notify both the old and new addresses
with directional language; the old-address copy explicitly warns that
whoever made the change can now receive password reset emails.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/controllers/application_controller.rb

@@ -14,6 +14,10 @@ class ApplicationController < ActionController::Base
 
   private
 
+  def security_event_context
+    {ip: request.remote_ip, user_agent: request.user_agent, occurred_at: Time.current}
+  end
+
   # Remove a query parameter from a URL using proper URI parsing
   # More robust than regex - handles URL encoding, edge cases, etc.
   #