Next.js App Router, specify revalidation on server side functions that don't use 'fetch'

I am using the new App Router in my Next.js project.
My pages are filled with content from a Markdown file fetched from a bucket. On my Vercel hosting, these pages are cached forever, so changes to the Markdown file aren't shown.

The documentation shows how to specify cache expiration time, within the fetch parameters. But my functions does not use fetch, it uses the Firebase SDK.

I've also tried adding an extra step of creating an internal api route that returns the file's contents, and then the page route could fetch that api route, specifying the proper revalidation parameters. But Next.js does not seem to allow fetching internal api routes from within server side.

Below is the code for one of my routes, where downloadFromStorage returns the contents from the file in the bucket.

import {TextPage} from "@/components";
import {downloadFromStorage} from "@/app/api/file-handling";

export default async function PageTerms(props) {
    const text=await downloadFromStorage('pages/terms.md')
    return (<TextPage text={text.toString()}/>)
}

(the downloadFromStorage function)

import {storage} from "./firebase"
export async function downloadFromStorage(filename, bucketName = BUCKET) {
    const bucket = storage.bucket(bucketName);
    const file = bucket.file(filename);
    return new Promise((resolve, reject) => {
        file.download((err, contents) => {
            if (err) {
                reject(err);
                return;
            }
            resolve(contents);
        });
    });
}

You can add a route segment configuration with the revalidate option. This will then revalidate the entire segment based on the exported integer.

// revalidate every 60 seconds
export const revalidate = 60;

You can learn more about segment configurations and the revalidate option in the official documentation.