refactor(server/middleware): create middleware to add the current story's chapter index to the context as an int (if it exists)

because convenience

server/middleware/20.cidx.ts

@@ -0,0 +1,4 @@
+export default eventHandler((ev) => {
+	let chapParam = getRouterParams(ev).chapter;
+	ev.context.chapterIndex = !!chapParam ? parseInt(chapParam) - 1 : 0;
+});