fix: resolve Next.js 16 build errors

- Extract Elysia app from route.ts into lib/api/app.ts to avoid
  Next.js 16 invalid export validation ('app' is not a valid route
  export field)
- Remove runtime='nodejs' from opengraph-image.tsx (invalid segment
  config in Next.js 16)
- Replace computed revalidate expression in sitemap.ts with literal
  (Next.js cannot statically evaluate BinaryExpression for segment
  configs)
- Update eden.ts imports to match new app.ts location

app/sitemap.ts

@@ -13,7 +13,7 @@ import { fetchDynamicEntries } from "@/lib/sitemap/fetch-dynamic-entries"
  * On-demand purging is handled by `/api/revalidate-sitemap` which
  * calls `revalidatePath("/sitemap.xml")`.
  */
-export const revalidate = Number(process.env.SITEMAP_REVALIDATE_SECONDS) || 3600
+export const revalidate = 3600
 
 /**
  * Generates the sitemap for deckyvault.xyz.