New output kind that POSTs the current strip frame to a user-configured
HTTP endpoint, alongside WLED / MQTT / Hue. Stack mirrors the existing
output-target shape end-to-end: storage model + store, FastAPI router +
Pydantic schemas, JS feature module + modal template, router wiring in
api/__init__.py and the modal include in index.html. Tests cover both
the routes and the store.
