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>

crates/notifications/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "notifications"
+version.workspace = true
+edition.workspace = true
+
+[dependencies]
+anyhow.workspace = true
+reqwest.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+sqlx.workspace = true
+tokio.workspace = true
+tracing.workspace = true