Use Telegram sendPhoto API for conversion and metadata-approved events
when a book thumbnail is available on disk. Falls back to text message
if photo upload fails.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Add notifications crate shared between API and indexer to send Telegram
messages on scan/thumbnail/conversion completion/failure, metadata linking,
batch and refresh events. Configurable via a new Notifications tab in the
backoffice settings with per-event toggle switches grouped by category.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
