Mark sitemap route as force-static

Its content (fixed routes + blog post list) never needs per-request
dynamic execution, so pin it explicitly rather than relying on Next's
default inference.

src/app/sitemap.ts

@@ -1,6 +1,8 @@
 import { MetadataRoute } from "next";
 import { blogPosts, SITE_URL } from "@/data/blog";
 
+export const dynamic = "force-static";
+
 export default function sitemap(): MetadataRoute.Sitemap {
   const staticRoutes = [
     "",