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/api/Cargo.toml

@@ -15,6 +15,7 @@ futures = "0.3"
 image.workspace = true
 jpeg-decoder.workspace = true
 lru.workspace = true
+notifications = { path = "../../crates/notifications" }
 stripstream-core = { path = "../../crates/core" }
 parsers = { path = "../../crates/parsers" }
 rand.workspace = true