Add verified_at timestamp to use for spam prevention

User are marked as verified after a join code is redeemed. The user is
redirected to Users::VerificationsController, either:

- after submitting a valid magic link code,
- or immediately after redeeming the join code (if they're already
  authenticated with the correct identity)

Account owners are automatically verified when the account is
created (because they have already provided a magic link code at that
point).

This sets up for later commits that will backfill existing users and
require verification before sending notification emails.

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

test/controllers/join_codes_controller_test.rb

@@ -36,7 +36,7 @@ class JoinCodesControllerTest < ActionDispatch::IntegrationTest
     end
 
     assert_redirected_to session_magic_link_url(script_name: nil)
-    assert_equal new_users_join_url(script_name: @account.slug), session[:return_to_after_authenticating]
+    assert_equal new_users_verification_url(script_name: @account.slug), session[:return_to_after_authenticating]
   end
 
   test "create for existing identity" do
@@ -55,7 +55,7 @@ class JoinCodesControllerTest < ActionDispatch::IntegrationTest
     assert_redirected_to landing_url(script_name: @account.slug)
   end
 
-  test "create for signed-in identity without a user in the account redirects to user setup" do
+  test "create for signed-in identity without a user in the account redirects to verification" do
     identity = identities(:mike)
     sign_in_as :mike
 
@@ -67,6 +67,6 @@ class JoinCodesControllerTest < ActionDispatch::IntegrationTest
       end
     end
 
-    assert_redirected_to new_users_join_url(script_name: @account.slug)
+    assert_redirected_to new_users_verification_url(script_name: @account.slug)
   end
 end