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.lock

@@ -76,6 +76,7 @@ dependencies = [
  "image",
  "jpeg-decoder",
  "lru",
+ "notifications",
  "parsers",
  "rand 0.8.5",
  "regex",
@@ -1240,6 +1241,7 @@ dependencies = [
  "futures",
  "image",
  "jpeg-decoder",
+ "notifications",
  "num_cpus",
  "parsers",
  "reqwest",
@@ -1663,6 +1665,19 @@ dependencies = [
  "nom",
 ]
 
+[[package]]
+name = "notifications"
+version = "1.21.2"
+dependencies = [
+ "anyhow",
+ "reqwest",
+ "serde",
+ "serde_json",
+ "sqlx",
+ "tokio",
+ "tracing",
+]
+
 [[package]]
 name = "nu-ansi-term"
 version = "0.50.3"