feat: add sitemap image URL validation gate

lib/sitemap/validate-image-url.ts

@@ -0,0 +1,16 @@
+/**
+ * Validates an image URL for sitemap use.
+ * Returns `null` for invalid inputs, otherwise returns the trimmed HTTPS URL.
+ */
+export function validateImageUrl(url: string | null | undefined): string | null {
+  if (!url || typeof url !== "string") return null
+
+  const trimmed = url.trim()
+  if (trimmed.length === 0) return null
+
+  if (!trimmed.startsWith("https://")) return null
+
+  if (trimmed.length > 2048) return null
+
+  return trimmed
+}