feat: add Telegram notification system with granular event toggles

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>

apps/indexer/src/job.rs

@@ -328,6 +328,7 @@ pub async fn process_job(
         removed_files: 0,
         errors: 0,
         warnings: 0,
+        new_series: 0,
     };
 
     let mut total_processed_count = 0i32;