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>

Cargo.toml

@@ -3,6 +3,7 @@ members = [
   "apps/api",
   "apps/indexer",
   "crates/core",
+  "crates/notifications",
   "crates/parsers",
 ]
 resolver = "2"