SECURITY: Convert send_digest to a post request (#19746)

Co-authored-by: Isaac Janzen <isaac.janzen@discourse.org>

spec/requests/admin/email_controller_spec.rb

@@ -337,6 +337,19 @@ RSpec.describe Admin::EmailController do
     end
   end
 
+  describe '#send_digest' do
+    context "when logged in as an admin" do
+      before { sign_in(admin) }
+
+      it "sends the digest" do
+        post "/admin/email/send-digest.json", params: {
+          last_seen_at: 1.week.ago, username: admin.username, email: email('previous_replies')
+        }
+        expect(response.status).to eq(200)
+      end
+    end
+  end
+
   describe '#handle_mail' do
     context "when logged in as an admin" do
       before { sign_in(admin) }