Not every notification is clickable, so click_url is optional end to end
(null/blank on the model, required=False on the serializer). When set, it's
merged into Gotify's own client::notification.click.url extras convention
without disturbing any other extras a producer already sends, so official
Gotify clients can open it on tap. Wired through both message-creation
paths: the single-push REST API and the bulk Excel upload. Also fixes a
pre-existing NameError in the bulk path (json.loads(extras) referenced an
undefined name instead of extras_str).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
