Adds Pushover as a new notification channel, allowing users to receive push notifications on their devices via the Pushover API. Backend: provider, type registration, validation, service wiring Frontend: form UI with app token and user key fields, validation, i18n Tests: full unit test suite (15 tests, 100% coverage on provider) Closes #3445 Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>