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/users/verifications_controller_test.rb

@@ -0,0 +1,24 @@
+require "test_helper"
+
+class Users::VerificationsControllerTest < ActionDispatch::IntegrationTest
+  test "new renders the auto-submit form" do
+    sign_in_as :david
+
+    get new_users_verification_path
+
+    assert_response :ok
+  end
+
+  test "create verifies the user and redirects to join" do
+    sign_in_as :david
+
+    user = users(:david)
+    user.update_column(:verified_at, nil)
+    assert_not user.verified?
+
+    post users_verifications_path
+
+    assert_redirected_to new_users_join_path
+    assert user.reload.verified?
+  end
+end