feat(seo): add sitemap generator

app/(app)/letters/[slug]/page.tsx

@@ -10,6 +10,8 @@ import { Card, CardContent } from "@/components/ui/card";
 import { buttonVariants } from "@/components/ui/button";
 import { ChevronLeft, ChevronRight } from "lucide-react";
 import { cn } from "@/lib/utils";
+import { JsonLd } from "@/components/seo/json-ld";
+import { articleJsonLd } from "@/lib/jsonld";
 
 export async function generateMetadata({
   params,
@@ -54,6 +56,15 @@ export default async function LetterPage({
 
   return (
     <div className="min-h-screen bg-background">
+      <JsonLd
+        data={articleJsonLd({
+          path: `/letters/${slug}`,
+          headline: `Letter to ${letter.recipientDisplay}`,
+          description: letter.paragraphs[0],
+          datePublished: letter.year ? String(letter.year) : undefined,
+          isPartOf: "Letters of Śrīla Prabhupāda",
+        })}
+      />
       <PageHeader sticky>
         <Link
           href={backHref}