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/src/telegram.rs

@@ -0,0 +1,46 @@
+use axum::{extract::State, Json};
+use serde::Serialize;
+use utoipa::ToSchema;
+
+use crate::{error::ApiError, state::AppState};
+
+#[derive(Serialize, ToSchema)]
+pub struct TelegramTestResponse {
+    pub success: bool,
+    pub message: String,
+}
+
+/// Test Telegram connection by sending a test message
+#[utoipa::path(
+    get,
+    path = "/telegram/test",
+    tag = "notifications",
+    responses(
+        (status = 200, body = TelegramTestResponse),
+        (status = 400, description = "Telegram not configured"),
+        (status = 401, description = "Unauthorized"),
+    ),
+    security(("Bearer" = []))
+)]
+pub async fn test_telegram(
+    State(state): State<AppState>,
+) -> Result<Json<TelegramTestResponse>, ApiError> {
+    let config = notifications::load_telegram_config(&state.pool)
+        .await
+        .ok_or_else(|| {
+            ApiError::bad_request(
+                "Telegram is not configured or disabled. Set bot_token, chat_id, and enable it.",
+            )
+        })?;
+
+    match notifications::send_test_message(&config).await {
+        Ok(()) => Ok(Json(TelegramTestResponse {
+            success: true,
+            message: "Test message sent successfully".to_string(),
+        })),
+        Err(e) => Ok(Json(TelegramTestResponse {
+            success: false,
+            message: format!("Failed to send: {e}"),
+        })),
+    }
+}