Add mark as new API endpoint for notifications

POST /api/notifications/{id}/mark_as_new sets is_new=1, allowing a notification to be flagged as unread after it has been read.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/web_server.rs

@@ -5,7 +5,7 @@ use axum::extract::Request;
 use axum::http::StatusCode;
 use axum::middleware::Next;
 use axum::response::Response;
-use axum::routing::{delete, get, put};
+use axum::routing::{delete, get, post, put};
 use axum::{Router, http};
 use log::{debug, error, info};
 use tower::ServiceBuilder;
@@ -38,6 +38,10 @@ pub async fn serve() -> Result<(), Box<dyn Error>> {
             "/api/notifications/{id}/read_without_flagging",
             get(notifications::read_without_flagging),
         )
+        .route(
+            "/api/notifications/{id}/mark_as_new",
+            post(notifications::mark_as_new),
+        )
         .route_layer(axum::middleware::from_fn(auth))
         .layer(ServiceBuilder::new().layer(cors_layer))
         .route(